I made also several experiments on the action of the vapour of the carbonate on the glands; but will give only a few cases. The cut end of the footstalk of a young leaf was protected with sealing-wax, and was then placed under a small bell-glass, with a large pinch of the carbonate. After 10 m. the glands showed a considerable degree of aggregation, and the protoplasm lining the cells of the pedicels was a little separated from the walls. Another leaf was left for 50 m. with the same result, excepting that the hairs became throughout their whole length of a brownish colour. In a third leaf, which was exposed for 1 hr. 50 m., there was much aggregated matter in the glands; and some of the masses showed signs of breaking up into brown granular matter. This leaf was again placed in the vapour, so that it was exposed altogether for 5 hrs. 30 m.; and now, though I examined a large number of glands, aggregated masses were found in only two or three; in all the others, the masses, which before had been globular, were converted into brown, opaque, granular matter. We thus see that exposure to the vapour for a considerable time produces the same effects as long immersion in a strong solution. In both cases there could hardly be a doubt that the salt had been absorbed chiefly or exclusively by the glands.

On another occasion bits of damp fibrin, drops of a weak infusion of raw meat and of water, were left for 24 hrs. on some leaves; the hairs were then examined, but to my surprise differed in no respect from others which had not been touched by these fluids. Most of the cells, however, included hyaline, motionless little spheres, which did not seem to consist of protoplasm, but, I suppose, of some balsam or essential oil.

Pelargonium zonale (var. edged with white).—The leaves [page 351] are clothed with numerous multicellular hairs; some simply pointed; others bearing glandular heads, and differing much in length. The glands on a piece of leaf were examined and found to contain only limpid fluid; most of the water was removed from beneath the covering glass, and a minute drop of one part of carbonate of ammonia to 146 of water was added; so that an extremely small dose was given. After an interval of only 3 m. there were signs of aggregation within the glands of the shorter hairs; and after 5 m. many small globules of a pale brown tint appeared in all of them; similar globules, but larger, being found in the large glands of the longer hairs. After the specimen had been left for 1 hr. in the solution, many of the smaller globules had changed their positions; and two or three vacuoles or small spheres (for I know not which they were) of a rather darker tint appeared within some of the larger globules. Little globules could now be seen in some of the uppermost cells of the pedicels, and the protoplasmic lining was slightly separated from the walls of the lower cells. After 2 hrs. 30 m. from the time of first immersion, the large globules within the glands of the longer hairs were converted into masses of darker brown granular matter. Hence from what we have seen with Primula sinensis, there can be little doubt that these masses originally consisted of living protoplasm.

A drop of a weak infusion of raw meat was placed on a leaf, and after 2 hrs. 30 m. many spheres could be seen within the glands. These spheres, when looked at again after 30 m., had slightly changed their positions and forms, and one had separated into two; but the changes were not quite like those which the protoplasm of Drosera undergoes. These hairs, moreover, had not been examined before immersion, and there were similar spheres in some glands which had not been touched by the infusion.

Erica tetralix.—A few long glandular hairs project from the margins of the upper surfaces of the leaves. The pedicels are formed of several rows of cells, and support rather large globular heads, secreting viscid matter, by which minute insects are occasionally, though rarely, caught. Some leaves were left for 23 hrs. in a weak infusion of raw meat and in water, and the hairs were then compared, but they differed very little or not at all. In both cases the contents of the cells seemed rather more granular than they were before; but the granules did not exhibit any movement. Other leaves were left for 23 hrs. in a solution of one part of carbonate of ammonia to 218 of water, and here again the granular matter appeared to have increased [page 352] in amount; but one such mass retained exactly the same form as before after an interval of 5 hrs., so that it could hardly have consisted of living protoplasm. These glands seem to have very little or no power of absorption, certainly much less than those of the foregoing plants.

Mirabilis longiflora.—The stems and both surfaces of the leaves bear viscid hairs. young plants, from 12 to 18 inches in height in my greenhouse, caught so many minute Diptera, Coleoptera, and larvae, that they were quite dusted with them. The hairs are short, of unequal lengths, formed of a single row of cells, surmounted by an enlarged cell which secretes viscid matter. These terminal cells or glands contain granules and often globules of granular matter. Within a gland which had caught a small insect, one such mass was observed to undergo incessant changes of form, with the occasional appearance of vacuoles. But I do not believe that this protoplasm had been generated by matter absorbed from the dead insect; for, on comparing several glands which had and had not caught insects, not a shade of difference could be perceived between them, and they all contained fine granular matter. A piece of leaf was immersed for 24 hrs. in a solution of one part of carbonate of ammonia to 218 of water, but the hairs seemed very little affected by it, excepting that perhaps the glands were rendered rather more opaque. In the leaf itself, however, the grains of chlorophyll near the cut surfaces had run together, or become aggregated. Nor were the glands on another leaf, after an immersion for 24 hrs. in an infusion of raw meat, in the least affected; but the protoplasm lining the cells of the pedicels had shrunk greatly from the walls. This latter effect may have been due to exosmose, as the infusion was strong. We may, therefore, conclude that the glands of this plant either have no power of absorption or that the protoplasm which they contain is not acted on by a solution of carbonate of ammonia (and this seems scarcely credible) or by an infusion of meat.

Nicotiana tabacum.—This plant is covered with innumerable hairs of unequal lengths, which catch many minute insects. The pedicels of the hairs are divided by transverse partitions, and the secreting glands are formed of many cells, containing greenish matter with little globules of some substance. Leaves were left in an infusion of raw meat and in water for 26 hrs., but presented no difference. Some of these same leaves were then left for above 2 hrs. in a solution of carbonate of ammonia, but no effect was produced. I regret that other experiments were not tried with more care, as M. Schloesing [page 353] has shown* that tobacco plants supplied with the vapour of carbonate of ammonia yield on analysis a greater amount of nitrogen than other plants not thus treated; and, from what we have seen, it is probable that some of the vapour may be absorbed by the glandular hairs.]

Summary of the Observations on Glandular Hairs.—From the foregoing observations, few as they are, we see that the glands of two species of Saxifraga, of a Primula and Pelargonium, have the power of rapid absorption; whereas the glands of an Erica, Mirabilis, and Nicotiana, either have no such power, or the contents of the cells are not affected by the fluids employed, namely a solution of carbonate of ammonia and an infusion of raw meat. As the glands of the Mirabilis contain protoplasm, which did not become aggregated from exposure to the fluids just named, though the contents of the cells in the blade of the leaf were greatly affected by carbonate of ammonia, we may infer that they cannot absorb. We may further infer that the innumerable insects caught by this plant are of no more service to it than are those which adhere to the deciduous and sticky scales of the leaf-buds of the horse-chestnut.

The most interesting case for us is that of the two species of Saxifraga, as this genus is distantly allied to Drosera. Their glands absorb matter from an infusion of raw meat, from solutions of the nitrate and carbonate of ammonia, and apparently from decayed insects. This was shown by the changed dull purple colour of the protoplasm within the cells of the glands, by its state of aggregation, and apparently by its more rapid spontaneous movements.

* 'Comptes rendus,' June 15, 1874. A good abstract of this paper is given in the 'Gardener's Chronicle,' July 11, 1874. [page 354]

The aggregating process spreads from the glands down the pedicels of the hairs; and we may assume that any matter which is absorbed ultimately reaches the tissues of the plant. On the other hand, the process travels up the hairs whenever a surface is cut and exposed to a solution of the carbonate of ammonia.

The glands on the flower-stalks and leaves of Primula sinensis quickly absorb a solution of the carbonate of ammonia, and the protoplasm which they contain becomes aggregated. The process was seen in some cases to travel from the glands into the upper cells of the pedicels. Exposure for 10 m. to the vapour of this salt likewise induced aggregation. When leaves were left from 6 hrs. to 7 hrs. in a strong solution, or were long exposed to the vapour, the little masses of protoplasm became disintegrated, brown, and granular, and were apparently killed. An infusion of raw meat produced no effect on the glands.

The limpid contents of the glands of Pelargonium zonale became cloudy and granular in from 3 m. to 5 m. when they were immersed in a weak solution of the carbonate of ammonia; and in the course of 1 hr. granules appeared in the upper cells of the pedicels. As the aggregated masses slowly changed their forms, and as they suffered disintegration when left for a considerable time in a strong solution, there can be little doubt that they consisted of protoplasm. It is doubtful whether an infusion of raw meat produced any effect.

The glandular hairs of ordinary plants have generally been considered by physiologists to serve only as secreting or excreting organs, but we now know that they have the power, at least in some cases, of absorbing both a solution and the vapour of ammonia. As rain-water contains a small percentage of ammonia, and the atmosphere a minute quantity of the carbonate, this [page 355] power can hardly fail to be beneficial. Nor can the benefit be quite so insignificant as it might at first be thought, for a moderately fine plant of Primula sinensis bears the astonishing number of above two millions and a half of glandular hairs,* all of which are able to absorb ammonia brought to them by the rain. It is moreover probable that the glands of some of the above named plants obtain animal matter from the insects which are occasionally entangled by the viscid secretion.

CONCLUDING REMARKS ON THE DROSERACEAE.

The six known genera composing this family have now been described in relation to our present subject, as far as my means have permitted. They all capture insects. This is effected by Drosophyllum, Roridula, and Byblis, solely by the viscid fluid secreted from their glands; by Drosera, through the same means, together with the movements of the tentacles; by Dionaea and Aldrovanda, through the closing of the blades of the leaf. In these two last genera rapid

* My son Francis counted the hairs on a space measured by means of a micrometer, and found that there were 35,336 on a square inch of the upper surface of a leaf, and 30,035 on the lower surface; that is, in about the proportion of 100 on the upper to 85 on the lower surface. On a square inch of both surfaces there were 65,371 hairs. A moderately fine plant bearing twelve leaves (the larger ones being a little more than 2 inches in diameter) was now selected, and the area of all the leaves, together with their foot-stalks (the flower-stems not being included), was found by a planimeter to be 39.285 square inches; so that the area of both surfaces was 78.57 square inches. Thus the plant (excluding the flower-stems) must have borne the astonishing number of 2,568,099 glandular hairs. The hairs were counted late in the autumn, and by the following spring (May) the leaves of some other plants of the same lot were found to be from one-third to one-fourth broader and longer than they were before; so that no doubt the glandular hairs had increased in number, and probably now much exceeded three millions. [page 356]

movement makes up for the loss of viscid secretion. In every case it is some part of the leaf which moves. In Aldrovanda it appears to be the basal parts alone which contract and carry with them the broad, thin margins of the lobes. In Dionaea the whole lobe, with the exception of the marginal prolongations or spikes, curves inwards, though the chief seat of movement is near the midrib. In Drosera the chief seat is in the lower part of the tentacles, which, homologically, may be considered as prolongations of the leaf; but the whole blade often curls inwards, converting the leaf into a temporary stomach.

There can hardly be a doubt that all the plants belonging to these six genera have the power of dissolving animal matter by the aid of their secretion, which contains an acid, together with a ferment almost identical in nature with pepsin; and that they afterwards absorb the matter thus digested. This is certainly the case with Drosera, Drosophyllum, and Dionaea; almost certainly with Aldrovanda; and, from analogy, very probable with Roridula and Byblis. We can thus understand how it is that the three first-named genera are provided with such small roots, and that Aldrovanda is quite rootless; about the roots of the two other genera nothing is known. It is, no doubt, a surprising fact that a whole group of plants (and, as we shall presently see, some other plants not allied to the Droseraceae) should subsist partly by digesting animal matter, and partly by decomposing carbonic acid, instead of exclusively by this latter means, together with the absorption of matter from the soil by the aid of roots. We have, however, an equally anomalous case in the animal kingdom; the rhizocephalous crustaceans do not feed like other animals by their mouths, for they are destitute of an [page 357] alimentary canal; but they live by absorbing through root-like processes the juices of the animals on which they are parasitic.*

Of the six genera, Drosera has been incomparably the most successful in the battle for life; and a large part of its success may be attributed to its manner of catching insects. It is a dominant form, for it is believed to include about 100 species, which range in the Old World from the Arctic regions to Southern India, to the Cape of Good Hope, Madagascar, and Australia; and in the New World from Canada to Tierra del Fuego. In this respect it presents a marked contrast with the five other genera, which appear to be failing groups. Dionaea includes only a single species, which is confined to one district in Carolina. The three varieties or closely allied species of Aldrovanda, like so many water-plants, have a wide range from Central Europe to Bengal and Australia. Drosophyllum includes only one species, limited to Portugal and Morocco. Roridula and Byblis each have (as I

* Fritz Mller, 'Facts for Darwin, ' Eng. trans. 1869, p. 139. The rhizocephalous crustaceans are allied to the cirripedes. It is hardly possible to imagine a greater difference than that between an animal with prehensile limbs, a well-constructed mouth and alimentary canal, and one destitute of all these organs and feeding by absorption through branching root-like processes. If one rare cirripede, the Anelasma squalicola, had become extinct, it would have been very difficult to conjecture how so enormous a change could have been gradually effected. But, as Fritz Mller remarks, we have in Anelasma an animal in an almost exactly intermediate condition, for it has root-like processes embedded in the skin of the shark on which it is parasitic, and its prehensile cirri and mouth (as described in my monograph on the Lepadidae, 'Ray Soc.' 1851, p. 169) are in a most feeble and almost rudimentary condition. Dr. R. Kossmann has given a very interesting discussion on this subject in his 'Suctoria and Lepadidae,' 1873. See also, Dr. Dohrn, 'Der Ursprung der Wirbelthiere,' 1875, p. 77.

Bentham and Hooker, 'Genera Plantarum.' Australia is the metropolis of the genus, forty-one species having been described from this country, as Prof. Oliver informs me. [page 358]

hear from Prof. Oliver) two species; the former confined to the western parts of the Cape of Good Hope, and the latter to Australia. It is a strange fact that Dionaea, which is one of the most beautifully adapted plants in the vegetable kingdom, should apparently be on the high-road to extinction. This is all the more strange as the organs of Dionaea are more highly differentiated than those of Drosera; its filaments serve exclusively as organs of touch, the lobes for capturing insects, and the glands, when excited, for secretion as well as for absorption; whereas with Drosera the glands serve all these purposes, and secrete without being excited.

By comparing the structure of the leaves, their degree of complication, and their rudimentary parts in the six genera, we are led to infer that their common parent form partook of the characters of Drosophyllum, Roridula, and Byblis. The leaves of this ancient form were almost certainly linear, perhaps divided, and bore on their upper and lower surfaces glands which had the power of secreting and absorbing. Some of these glands were mounted on pedicels, and others were almost sessile; the latter secreting only when stimulated by the absorption of nitrogenous matter. In Byblis the glands consist of a single layer of cells, supported on a unicellular pedicel; in Roridula they have a more complex structure, and are supported on pedicels formed of several rows of cells; in Drosophyllum they further include spiral cells, and the pedicels include a bundle of spiral vessels. But in these three genera these organs do not possess any power of movement, and there is no reason to doubt that they are of the nature of hairs or trichomes. Although in innumerable instances foliar organs move when excited, no case is known of a trichome having such [page 359] power.* We are thus led to inquire how the so-called tentacles of Drosera, which are manifestly of the same general nature as the glandular hairs of the above three genera, could have acquired the power of moving. Many botanists maintain that these tentacles consist of prolongations of the leaf, because they include vascular tissue, but this can no longer be considered as a trustworthy distinction. The possession of the power of movement on excitement would have been safer evidence. But when we consider the vast number of the tentacles on both surfaces of the leaves of Drosophyllum, and on the upper surface of the leaves of Drosera, it seems scarcely possible that each tentacle could have aboriginally existed as a prolongation of the leaf. Roridula, perhaps, shows us how we may reconcile these difficulties with respect to the homological nature of the tentacles. The lateral divisions of the leaves of this plant terminate in long tentacles; and these include spiral vessels which extend for only a short distance up them, with no line of demarcation between what is plainly the prolongation of the leaf and the pedicel of a glandular hair. Therefore there would be nothing anomalous or unusual in the basal parts of these tentacles, which correspond with the marginal ones of Drosera, acquiring the power of movement; and we know that in Drosera it is only the lower part which becomes inflected. But in order to understand how in this latter genus not only the marginal but all the inner tentacles have become capable of movement, we must further assume, either that through the principle of correlated development this

* Sachs, 'Trait de Botanique' 3rd edit. 1874, p. 1026.

Dr. Warming 'Sur la Diffrence entres les Trichomes,' Copenhague, 1873, p. 6. 'Extrait des Videnskabelige Meddelelser de la Soc. d'Hist. nat. de Copenhague,' Nos. 10-12, 1872. [page 360]

power was transferred to the basal parts of the hairs, or that the surface of the leaf has been prolonged upwards at numerous points, so as to unite with the hairs, thus forming the bases of the inner tentacles.

The above named three genera, namely Drosophyllum, Roridula, and Byblis, which appear to have retained a primordial condition, still bear glandular hairs on both surfaces of their leaves; but those on the lower surface have since disappeared in the more highly developed genera, with the partial exception of one species, Drosera binata. The small sessile glands have also disappeared in some of the genera, being replaced in Roridula by hairs, and in most species of Drosera by absorbent papillae. Drosera binata, with its linear and bifurcating leaves, is in an intermediate condition. It still bears some sessile glands on both surfaces of the leaves, and on the lower surface a few irregularly placed tentacles, which are incapable of movement. A further slight change would convert the linear leaves of this latter species into the oblong leaves of Drosera anglica, and these might easily pass into orbicular ones with footstalks, like those of Drosera rotundifolia. The footstalks of this latter species bear multicellular hairs, which we have good reason to believe represent aborted tentacles.

The parent form of Dionaea and Aldrovanda seems to have been closely allied to Drosera, and to have had rounded leaves, supported on distinct footstalks, and furnished with tentacles all round the circumference, with other tentacles and sessile glands on the upper surface. I think so because the marginal spikes of Dionaea apparently represent the extreme marginal tentacles of Drosera, the six (sometimes eight) sensitive filaments on the upper surface, as well as the more numerous ones in Aldrovanda, representing the central [page 361] tentacles of Drosera, with their glands aborted, but their sensitiveness retained. Under this point of view we should bear in mind that the summits of the tentacles of Drosera, close beneath the glands, are sensitive.

The three most remarkable characters possessed by the several members of the Droseraceae consist in the leaves of some having the power of movement when excited, in their glands secreting a fluid which digests animal matter, and in their absorption of the digested matter. Can any light be thrown on the steps by which these remarkable powers were gradually acquired?

As the walls of the cells are necessarily permeable to fluids, in order to allow the glands to secrete, it is not surprising that they should readily allow fluids to pass inwards; and this inward passage would deserve to be called an act of absorption, if the fluids combined with the contents of the glands. Judging from the evidence above given, the secreting glands of many other plants can absorb salts of ammonia, of which they must receive small quantities from the rain. This is the case with two species of Saxifraga, and the glands of one of them apparently absorb matter from captured insects, and certainly from an infusion of raw meat. There is, therefore, nothing anomalous in the Droseraceae having acquired the power of absorption in a much more highly developed degree.

It is a far more remarkable problem how the members of this family, and Pinguicula, and, as Dr. Hooker has recently shown, Nepenthes, could all have acquired the power of secreting a fluid which dissolves or digests animal matter. The six genera of the Droseraceae very probably inherited this power from a common progenitor, but this cannot apply to [page 362] Pinguicula or Nepenthes, for these plants are not at all closely related to the Droceraceae. But the difficulty is not nearly so great as it at first appears. Firstly, the juices of many plants contain an acid, and, apparently, any acid serves for digestion. Secondly, as Dr. Hooker has remarked in relation to the present subject in his address at Belfast (1874), and as Sachs repeatedly insists,* the embryos of some plants secrete a fluid which dissolves albuminous substances out of the endosperm; although the endosperm is not actually united with, only in contact with, the embryo. All plants, moreover, have the power of dissolving albuminous or proteid substances, such as protoplasm, chlorophyll, gluten, aleurone, and of carrying them from one part to other parts of their tissues. This must be effected by a solvent, probably consisting of a ferment together with an acid. Now, in the case of plants which are able to absorb already soluble matter from captured insects, though not capable of true digestion, the solvent just referred to, which must be occasionally present in the glands, would be apt to exude from the glands together with the viscid secretion, inasmuch as endosmose is accompanied by exosmose. If such exudation did ever occur, the solvent would act on the animal matter contained within the captured insects, and this would be an act of true digestion. As it cannot be doubted that this process would be of high service to plants

* 'Trait de Botanique' 3rd edit. 1874, p. 844. See also for following facts pp. 64, 76, 828, 831.

Since this sentence was written, I have received a paper by Gorup-Besanez ('Berichte der Deutschen Chem. Gesellschaft,' Berlin, 1874, p. 1478), who, with the aid of Dr. H. Will, has actually made the discovery that the seeds of the vetch contain a ferment, which, when extracted by glycerine, dissolves albuminous substances, such as fibrin, and converts them into true peptones. [page 363]

growing in very poor soil, it would tend to be perfected through natural selection. Therefore, any ordinary plant having viscid glands, which occasionally caught insects, might thus be converted under favourable circumstances into a species capable of true digestion. It ceases, therefore, to be any great mystery how several genera of plants, in no way closely related together, have independently acquired this same power.

As there exist several plants the glands of which cannot, as far as is known, digest animal matter, yet can absorb salts of ammonia and animal fluids, it is probable that this latter power forms the first stage towards that of digestion. It might, however, happen, under certain conditions, that a plant, after having acquired the power of digestion, should degenerate into one capable only of absorbing animal matter in solution, or in a state of decay, or the final products of decay, namely the salts of ammonia. It would appear that this has actually occurred to a partial extent with the leaves of Aldrovanda; the outer parts of which possess absorbent organs, but no glands fitted for the secretion of any digestive fluid, these being confined to the inner parts.

Little light can be thrown on the gradual acquirement of the third remarkable character possessed by the more highly developed genera of the Droseraceae, namely the power of movement when excited. It should, however, be borne in mind that leaves and their homologues, as well as flower-peduncles, have gained this power, in innumerable instances, independently of inheritance from any common parent form; for instance, in tendril-bearers and leaf-climbers (i.e. plants with their leaves, petioles and flower-peduncles, &c., modified for prehension) belonging to a large [page 364] number of the most widely distinct orders,—in the leaves of the many plants which go to sleep at night, or move when shaken,—and in the irritable stamens and pistils of not a few species. We may therefore infer that the power of movement can be by some means readily acquired. Such movements imply irritability or sensitiveness, but, as Cohn has remarked,* the tissues of the plants thus endowed do not differ in any uniform manner from those of ordinary plants; it is therefore probable that all leaves are to a slight degree irritable. Even if an insect alights on a leaf, a slight molecular change is probably transmitted to some distance across its tissue, with the sole difference that no perceptible effect is produced. We have some evidence in favour of this belief, for we know that a single touch on the glands of Drosera does not excite inflection; yet it must produce some effect, for if the glands have been immersed in a solution of camphor, inflection follows within a shorter time than would have followed from the effects of camphor alone. So again with Dionaea, the blades in their ordinary state may be roughly touched without their closing; yet some effect must be thus caused and transmitted across the whole leaf, for if the glands have recently absorbed animal matter, even a delicate touch causes them to close instantly. On the whole we may conclude that the acquirement of a high degree of sensitiveness and of the power of movement by certain genera of the Droseraceae presents no greater difficulty than that presented by the similar but feebler powers of a multitude of other plants.

* See the abstract of his memoir on the contractile tissues of plants, in the 'Annals and Mag. of Nat. Hist.' 3rd series, vol. xi. p. 188.) [page 365]

The specialised nature of the sensitiveness possessed by Drosera and Dionaea, and by certain other plants, well deserves attention. A gland of Drosera may be forcibly hit once, twice, or even thrice, without any effect being produced, whilst the continued pressure of an extremely minute particle excites movement. On the other hand, a particle many times heavier may be gently laid on one of the filaments of Dionaea with no effect; but if touched only once by the slow movement of a delicate hair, the lobes close; and this difference in the nature of the sensitiveness of these two plants stands in manifest adaptation to their manner of capturing insects. So does the fact, that when the central glands of Drosera absorb nitrogenous matter, they transmit a motor impulse to the exterior tentacles much more quickly than when they are mechanically irritated; whilst with Dionaea the absorption of nitrogenous matter causes the lobes to press together with extreme slowness, whilst a touch excites rapid movement. Somewhat analogous cases may be observed, as I have shown in another work, with the tendrils of various plants; some being most excited by contact with fine fibres, others by contact with bristles, others with a flat or a creviced surface. The sensitive organs of Drosera and Dionaea are also specialised, so as not to be uselessly affected by the weight or impact of drops of rain, or by blasts of air. This may be accounted for by supposing that these plants and their progenitors have grown accustomed to the repeated action of rain and wind, so that no molecular change is thus induced; whilst they have been rendered more sensitive by means of natural selection to the rarer impact or pressure of solid bodies. Although the absorption by the glands of Drosera of various fluids excites move- [page 366] ment, there is a great difference in the action of allied fluids; for instance, between certain vegetable acids, and between citrate and phosphate of ammonia. The specialised nature and perfection of the sensitiveness in these two plants is all the more astonishing as no one supposes that they possess nerves; and by testing Drosera with several substances which act powerfully on the nervous system of animals, it does not appear that they include any diffused matter analogous to nerve-tissue.

Although the cells of Drosera and Dionaea are quite as sensitive to certain stimulants as are the tissues which surround the terminations of the nerves in the higher animals, yet these plants are inferior even to animals low down in the scale, in not being affected except by stimulants in contact with their sensitive parts. They would, however, probably be affected by radiant heat; for warm water excites energetic movement. When a gland of Drosera, or one of the filaments of Dionaea, is excited, the motor impulse radiates in all directions, and is not, as in the case of animals, directed towards special points or organs. This holds good even in the case of Drosera when some exciting substance has been placed at two points on the disc, and when the tentacles all round are inflected with marvellous precision towards the two points. The rate at which the motor impulse is transmitted, though rapid in Dionaea, is much slower than in most or all animals. This fact, as well as that of the motor impulse not being specially directed to certain points, are both no doubt due to the absence of nerves. Nevertheless we perhaps see the prefigurement of the formation of nerves in animals in the transmission of the motor impulse being so much more rapid down the confined space within the tentacles of Drosera than [page 367] elsewhere, and somewhat more rapid in a longitudinal than in a transverse direction across the disc. These plants exhibit still more plainly their inferiority to animals in the absence of any reflex action, except in so far as the glands of Drosera, when excited from a distance, send back some influence which causes the contents of the cells to become aggregated down to the bases of the tentacles. But the greatest inferiority of all is the absence of a central organ, able to receive impressions from all points, to transmit their effects in any definite direction, to store them up and reproduce them. [page 368]

CHAPTER XVI.

PINGUICULA.

Pinguicula vulgaris—Structure of leaves—Number of insects and other objects caught— Movement of the margins of the leaves—Uses of this movement—Secretion, digestion, and absorption—Action of the secretion on various animal and vegetable substances—The effects of substances not containing soluble nitrogenous matter on the glands—Pinguicula grandiflora—Pinguicula lusitanica, catches insects—Movement of the leaves, secretion and digestion.

PINGUICULA VULGARIS.—This plant grows in moist places, generally on mountains. It bears on an average eight, rather thick, oblong, light green leaves, having scarcely any footstalk. A full-sized leaf is about 1 1/2 inch in length and 3/4 inch in breadth. The young central leaves are deeply concave, and project upwards; the older ones towards the outside are flat or convex, and lie close to the ground, forming a rosette from 3 to 4 inches in diameter. The margins of the leaves are incurved. Their upper surfaces are thickly covered with two sets of glandular hairs, differing in the size of the glands and in the length of their pedicels. The larger glands have a circular outline as seen from above, and are of moderate thickness; they are divided by radiating partitions into sixteen cells, containing light-green, homogeneous fluid. They are supported on elongated, unicellular pedicels (containing a nucleus with a nucleolus) which rest on slight prominences. The small glands differ only in being formed of about half the number of cells, containing much paler fluid, and supported on much shorter pedicels. Near the midrib, towards the base of the leaf, the [page 369] pedicels are multicellular, are longer than elsewhere, and bear smaller glands. All the glands secrete a colourless fluid, which is so viscid that I have seen a fine thread drawn out to a length of 18 inches; but the fluid in this case was secreted by a gland which had been excited. The edge of the leaf is translucent, and does not bear any glands; and here the spiral vessels, proceeding from the midrib, terminate in cells marked by a spiral line, somewhat like those within the glands of Drosera.

The roots are short. Three plants were dug up in North Wales on June 20, and carefully washed; each bore five or six unbranched roots, the longest of which was only 1.2 of an inch. Two rather young plants were examined on September 28; these had a greater number of roots, namely eight and eighteen, all under 1 inch in length, and very little branched.

I was led to investigate the habits of this plant by being told by Mr. W. Marshall that on the mountains of Cumberland many insects adhere to the leaves.

[A friend sent me on June 23 thirty-nine leaves from North Wales, which were selected owing to objects of some kind adhering to them. Of these leaves, thirty-two had caught 142 insects, or on an average 4.4 per leaf, minute fragments of insects not being included. Besides the insects, small leaves belonging to four different kinds of plants, those of Erica tetralix being much the commonest, and three minute seedling plants, blown by the wind, adhered to nineteen of the leaves. One had caught as many as ten leaves of the Erica. Seeds or fruits, commonly of Carex and one of Juncus, besides bits of moss and other rubbish, likewise adhered to six of the thirty-nine leaves. The same friend, on June 27, collected nine plants bearing seventy-four leaves, and all of these, with the exception of three young leaves, had caught insects; thirty insects were counted on one leaf, eighteen on a second, and sixteen on a third. Another friend examined on August 22 some plants in Donegal, Ireland, and found insects on 70 out of 157 leaves; fifteen of [page 370] these leaves were sent me, each having caught on an average 2.4 insects. To nine of them, leaves (mostly of Erica tetralix) adhered; but they had been specially selected on this latter account. I may add that early in August my son found leaves of this same Erica and the fruits of a Carex on the leaves of a Pinguicula in Switzerland, probably Pinguicula alpina; some insects, but no great number, also adhered to the leaves of this plant, which had much better developed roots than those of Pinguicula vulgaris. In Cumberland, Mr. Marshall, on September 3, carefully examined for me ten plants bearing eighty leaves; and on sixty-three of these (i.e. on 79 per cent.) he found insects, 143 in number; so that each leaf had on an average 2.27 insects. A few days later he sent me some plants with sixteen seeds or fruits adhering to fourteen leaves. There was a seed on three leaves on the same plant. The sixteen seeds belonged to nine different kinds, which could not be recognised, excepting one of Ranunculus, and several belonging to three or four distinct species of Carex. It appears that fewer insects are caught late in the year than earlier; thus in Cumberland from twenty to twenty-four insects were observed in the middle of July on several leaves, whereas in the beginning of September the average number was only 2.27. Most of the insects, in all the foregoing cases, were Diptera, but with many minute Hymenoptera, including some ants, a few small Coleoptera, larvae, spiders, and even small moths.]

We thus see that numerous insects and other objects are caught by the viscid leaves; but we have no right to infer from this fact that the habit is beneficial to the plant, any more than in the before given case of the Mirabilis, or of the horse-chestnut. But it will presently be seen that dead insects and other nitrogenous bodies excite the glands to increased secretion; and that the secretion then becomes acid and has the power of digesting animal substances, such as albumen, fibrin, &c. Moreover, the dissolved nitrogenous matter is absorbed by the glands, as shown by their limpid contents being aggregated into slowly moving granular masses of protoplasm. The same results follow when insects are naturally captured, and as the plant lives in poor soil and has small roots, there can be no [page 371] doubt that it profits by its power of digesting and absorbing matter from the prey which it habitually captures in such large numbers. It will, however, be convenient first to describe the movements of the leaves.

Movements of the Leaves.—That such thick, large leaves as those of Pinguicula vulgarisshould have the power of curving inwards when excited has never even been suspected. It is necessary to select for experiment leaves with their glands secreting freely, and which have been prevented from capturing many insects; as old leaves, at least those growing in a state of nature, have their margins already curled so much inwards that they exhibit little power of movement, or move very slowly. I will first give in detail the more important experiments which were tried, and then make some concluding remarks.

[Experiment 1.—A young and almost upright leaf was selected, with its two lateral edges equally and very slightly incurved. A row of small flies was placed along one margin. When looked at next day, after 15 hrs., this margin, but not the other, was found folded inwards, like the helix of the human ear, to the breadth of 1/10 of an inch, so as to lie partly over the row of flies (fig. 15). The glands on which the flies rested, as well as those on the over-lapping margin which had been brought into contact with the flies, were all secreting copiously.

FIG. 15. (Pinguicula vulgaris.) Outline of leaf with left margin inflected over a row of small flies.

Experiment 2.—A row of flies was placed on one margin of a rather old leaf, which lay flat on the ground; and in this case the margin, after the same interval as before, namely 15 hrs., had only just begun to curl inwards; but so much secretion had been poured forth that the spoon-shaped tip of the leaf was filled with it.

Experiment 3.—Fragments of a large fly were placed close to the apex of a vigorous leaf, as well as along half one margin. [page 372] After 4 hrs. 20 m. there was decided incurvation, which increased a little during the afternoon, but was in the same state on the following morning. Near the apex both margins were inwardly curved. I have never seen a case of the apex itself being in the least curved towards the base of the leaf. After 48 hrs. (always reckoning from the time when the flies were placed on the leaf) the margin had everywhere begun to unfold.

Experiment 4.—A large fragment of a fly was placed on a leaf, in a medial line, a little beneath the apex. Both lateral margins were perceptibly incurved in 3 hrs., and after 4 hrs. 20 m. to such a degree that the fragment was clasped by both margins. After 24 hrs. the two infolded edges near the apex (for the lower part of the leaf was not at all affected) were measured and found to be .11 of an inch (2.795 mm.) apart. The fly was now removed, and a stream of water poured over the leaf so as to wash the surface; and after 24 hrs. the margins were .25 of an inch (6.349 mm.) apart, so that they were largely unfolded. After an additional 24 hrs. they were completely unfolded. Another fly was now put on the same spot to see whether this leaf, on which the first fly had been left 24 hrs., would move again; after 10 hrs. there was a trace of incurvation, but this did not increase during the next 24 hrs. A bit of meat was also placed on the margin of a leaf, which four days previously had become strongly incurved over a fragment of a fly and had afterwards re-expanded; but the meat did not cause even a trace of incurvation. On the contrary, the margin became somewhat reflexed, as if injured, and so remained for the three following days, as long as it was observed.

Experiment 5.—A large fragment of a fly was placed halfway between the apex and base of a leaf and halfway between the midrib and one margin. A short space of this margin, opposite the fly, showed a trace of incurvation after 3 hrs., and this became strongly pronounced in 7 hrs. After 24 hrs. the infolded edge was only .16 of an inch (4.064 mm.) from the midrib. The margin now began to unfold, though the fly was left on the leaf; so that by the next morning (i.e. 48 hrs. from the time when the fly was first put on) the infolded edge had almost completely recovered its original position, being now .3 of an inch (7.62 mm.), instead of .16 of an inch, from the midrib. A trace of flexure was, however, still visible.

Experiment 6.—A young and concave leaf was selected with its margins slightly and naturally incurved. Two rather large, oblong, rectangular pieces of roast meat were placed with their ends touching the infolded edge, and .46 of an inch (11.68 mm.) [page 373] apart from one another. After 24 hrs. the margin was greatly and equally incurved (see fig. 16) throughout this space, and for a length of .12 or .13 of an inch (3.048 or 3.302 mm.) above and below each bit; so that the margin had been affected over a greater length between the two bits, owing to their conjoint action, than beyond them. The bits of meat were too large to be clasped by the margin, but they were tilted up, one of them so as to stand almost vertically. After 48 hrs. the margin was almost unfolded, and the bits had sunk down. When again examined after two days, the margin was quite unfolded, with the exception of the naturally inflected edge; and one of the bits of meat, the end of which had at first touched the edge, was now .067 of an inch (1.70 mm.) distant from it; so that this bit had been pushed thus far across the blade of the leaf.

FIG. 16. (Pinguicula vulgaris.) Outline of leaf, with right margin inflected against two square bits of meat.

Experiment 7.—A bit of meat was placed close to the incurved edge of a rather young leaf, and after it had re-expanded, the bit was left lying .11 of an inch (2.795 mm.) from the edge. The distance from the edge to the midrib of the fully expanded leaf was .35 of an inch (8.89 mm.); so that the bit had been pushed inwards and across nearly one-third of its semi-diameter.

Experiment 8.—Cubes of sponge, soaked in a strong infusion of raw meat, were placed in close contact with the incurved edges of two leaves,—an older and younger one. The distance from the edges to the midribs was carefully measured. After 1 hr. 17 m. there appeared to be a trace of incurvation. After 2 hrs. 17 m. both leaves were plainly inflected; the distance between the edges and midribs being now only half what it was at first. The incurvation increased slightly during the next 4 1/2 hrs., but remained nearly the same for the next 17 hrs. 30 m. In 35 hrs. from the time when the sponges were placed on the leaves, the margins were a little unfolded—to a greater degree in the younger than in the older leaf. The latter was not quite unfolded until the third day, and now both bits of sponge were left at the distance of .1 of an inch (2.54 mm.) from the edges; or about a quarter of the distance between the edge and midrib. A third bit of sponge adhered to the edge, and, as the margin unfolded, was dragged backwards, into its original position. [page 374]

Experiment 9.—A chain of fibres of roast meat, as thin as bristles and moistened with saliva, were placed down one whole side, close to the narrow, naturally incurved edge of a leaf. In 3 hrs. this side was greatly incurved along its whole length, and after 8 hrs. formed a cylinder, about 1/20 of an inch (1.27 mm) in diameter, quite concealing the meat. This cylinder remained closed for 32 hrs., but after 48 hrs. was half unfolded, and in 72 hrs. was as open as the opposite margin where no meat had been placed. As the thin fibres of meat were completely overlapped by the margin, they were not pushed at all inwards, across the blade.

Experiment 10.—Six cabbage seeds, soaked for a night in water, were placed in a row close to the narrow incurved edge of a leaf. We shall hereafter see that these seeds yield soluble matter to the glands. In 2 hrs. 25 m. the margin was decidedly inflected; in 4 hrs. it extended over the seeds for about half their breadth, and in 7 hrs. over three-fourths of their breadth, forming a cylinder not quite closed along the inner side, and about .7 of an inch (1.778 mm.) in diameter. After 24 hrs. the inflection had not increased, perhaps had decreased. The glands which had been brought into contact with the upper surfaces of the seeds were now secreting freely. In 36 hrs. from the time when the seeds were put on the leaf the margin had greatly, and after 48 hrs. had completely, re-expanded. As the seeds were no longer held by the inflected margin, and as the secretion was beginning to fail, they rolled some way down the marginal channel.

Experiment 11.—Fragments of glass were placed on the margins of two fine young leaves. After 2 hrs. 30 m. the margin of one certainly became slightly incurved; but the inflection never increased, and disappeared in 16 hrs. 30 m. from the time when the fragments were first applied. With the second leaf there was a trace of incurvation in 2 hrs. 15 m., which became decided in 4 hrs. 30 m., and still more strongly pronounced in 7 hrs., but after 19 hrs. 30 m. had plainly decreased. The fragments excited at most a slight and doubtful increase of the secretion; and in two other trials, no increase could be perceived. Bits of coal-cinders, placed on a leaf, produced no effect, either owing to their lightness or to the leaf being torpid.

Experiment 12.—We now turn to fluids. A row of drops of a strong infusion of raw meat were placed along the margins of two leaves; squares of sponge soaked in the same infusion being placed on the opposite margins. My object was to ascer- [page 375] tain whether a fluid would act as energetically as a substance yielding the same soluble matter to the glands. No distinct difference was perceptible; certainly none in the degree of incurvation; but the incurvation round the bits of sponge lasted rather longer, as might perhaps have been expected from the sponge remaining damp and supplying nitrogenous matter for a longer time. The margins, with the drops, became plainly incurved in 2 hrs. 17 m. The incurvation subsequently increased somewhat, but after 24 hrs. had greatly decreased.

Experiment 13.—Drops of the same strong infusion of raw meat were placed along the midrib of a young and rather deeply concave leaf. The distance across the broadest part of the leaf, between the naturally incurved edges, was .55 of an inch (13.97 mm.). In 3 hrs. 27 m. this distance was a trace less; in 6 hrs. 27 m. it was exactly .45 of an inch (11.43 mm.), and had therefore decreased by .1 of an inch (2.54 mm.). After only 10 hrs. 37 m. the margin began to re-expand, for the distance from edge to edge was now a trace wider, and after 24 hrs. 20 m. was as great, within a hair's breadth, as when the drops were first placed on the leaf. From this experiment we learn that the motor impulse can be transmitted to a distance of .22 of an inch (5.590 mm.) in a transverse direction from the midrib to both margins; but it would be safer to say .2 of an inch (5.08 mm.) as the drops spread a little beyond the midrib. The incurvation thus caused lasted for an unusually short time.

Experiment 14.—Three drops of a solution of one part of carbonate of ammonia to 218 of water (2 grs. to 1 oz.) were placed on the margin of a leaf. These excited so much secretion that in 1 h. 22 m. all three drops ran together; but although the leaf was observed for 24 hrs., there was no trace of inflection. We know that a rather strong solution of this salt, though it does not injure the leaves of Drosera, paralyses their power of movement, and I have no doubt, from the following case, that this holds good with Pinguicula.

Experiment 15.—A row of drops of a solution of one part of carbonate of ammonia to 875 of water (1 gr. to 2 oz.) was placed on the margin of a leaf. In 1 hr. there was apparently some slight incurvation, and this was well-marked in 3 hrs. 30 m. After 24 hrs. the margin was almost completely re-expanded.

Experiment 16.—A row of large drops of a solution of one part of phosphate of ammonia to 4375 of water (1 gr. to 10 oz.) was placed along the margin of a leaf. No effect was produced, and after 8 hrs. fresh drops were added along the same margin without the least effect. We know that a solution of this [page 376] strength acts powerfully on Drosera, and it is just possible that the solution was too strong. I regret that I did not try a weaker solution.

Experiment 17.—As the pressure from bits of glass causes incurvation, I scratched the margins of two leaves for some minutes with a blunt needle, but no effect was produced. The surface of a leaf beneath a drop of a strong infusion of raw meat was also rubbed for 10. m. with the end of a bristle, so as to imitate the struggles of a captured insect; but this part of the margin did not bend sooner than the other parts with undisturbed drops of the infusion.]

We learn from the foregoing experiments that the margins of the leaves curl inwards when excited by the mere pressure of objects not yielding any soluble matter, by objects yielding such matter, and by some fluids—namely an infusion of raw meat and a week solution of carbonate of ammonia. A stronger solution of two grains of this salt to an ounce of water, though exciting copious secretion, paralyses the leaf. Drops of water and of a solution of sugar or gum did not cause any movement. Scratching the surface of the leaf for some minutes produced no effect. Therefore, as far as we at present know, only two causes—namely slight continued pressure and the absorption of nitrogenous matter—excite movement. It is only the margins of the leaf which bend, for the apex never curves towards the base. The pedicels of the glandular hairs have no power of movement. I observed on several occasions that the surface of the leaf became slightly concave where bits of meat or large flies had long lain, but this may have been due to injury from over-stimulation.

The shortest time in which plainly marked movement was observed was 2 hrs. 17 m., and this occurred when either nitrogenous substances or fluids were placed on the leaves; but I believe that in some cases [page 377] there was a trace of movement in 1 hr. or 1 hr. 30 m. The pressure from fragments of glass excites movement almost as quickly as the absorption of nitrogenous matter, but the degree of incurvation thus caused is much less. After a leaf has become well incurved and has again expanded, it will not soon answer to a fresh stimulus. The margin was affected longitudinally, upwards or downwards, for a distance of .13 of an inch (3.302 mm.) from an excited point, but for a distance of .46 of an inch between two excited points, and transversely for a distance of .2 of an inch (5.08 mm.). The motor impulse is not accompanied, as in the case of Drosera, by any influence causing increased secretion; for when a single gland was strongly stimulated and secreted copiously, the surrounding glands were not in the least affected. The incurvation of the margin is independent of increased secretion, for fragments of glass cause little or no secretion, and yet excite movement; whereas a strong solution of carbonate of ammonia quickly excites copious secretion, but no movement.

One of the most curious facts with respect to the movement of the leaves is the short time during which they remain incurved, although the exciting object is left on them. In the majority of cases there was well-marked re-expansion within 24 hrs. from the time when even large pieces of meat, &c., were placed on the leaves, and in all cases within 48 hrs. In one instance the margin of a leaf remained for 32 hrs. closely inflected round thin fibres of meat; in another instance, when a bit of sponge, soaked in a strong infusion of raw meat, had been applied to a leaf, the margin began to unfold in 35 hrs. Fragments of glass keep the margin incurved for a shorter time than do nitrogenous bodies; for in the former case there was [page 378] complete re-expansion in 16 hrs. 30 m. Nitrogenous fluids act for a shorter time than nitrogenous substances; thus, when drops of an infusion of raw meat were placed on the midrib of a leaf, the incurved margins began to unfold in only 10 hrs. 37 m., and this was the quickest act of re-expansion observed by me; but it may have been partly due to the distance of the margins from the midrib where the drops lay.

We are naturally led to inquire what is the use of this movement which lasts for so short a time? If very small objects, such as fibres of meat, or moderately small objects, such as little flies or cabbage-seeds, are placed close to the margin, they are either completely or partially embraced by it. The glands of the overlapping margin are thus brought into contact with such objects and pour forth their secretion, afterwards absorbing the digested matter. But as the incurvation lasts for so short a time, any such benefit can be of only slight importance, yet perhaps greater than at first appears. The plant lives in humid districts, and the insects which adhere to all parts of the leaf are washed by every heavy shower of rain into the narrow channel formed by the naturally incurved edges. For instance, my friend in North Wales placed several insects on some leaves, and two days afterwards (there having been heavy rain in the interval) found some of them quite washed away, and many others safely tucked under the now closely inflected margins, the glands of which all round the insects were no doubt secreting. We can thus, also, understand how it is that so many insects, and fragments of insects, are generally found lying within the incurved margins of the leaves.

The incurvation of the margin, due to the presence of an exciting object, must be serviceable in another [page 379] and probably more important way. We have seen that when large bits of meat, or of sponge soaked in the juice of meat, were placed on a leaf, the margin was not able to embrace them, but, as it became incurved, pushed them very slowly towards the middle of the leaf, to a distance from the outside of fully .1 of an inch (2.54 mm.), that is, across between one-third and one-fourth of the space between the edge and midrib. Any object, such as a moderately sized insect, would thus be brought slowly into contact with a far larger number of glands, inducing much more secretion and absorption, than would otherwise have been the case. That this would be highly serviceable to the plant, we may infer from the fact that Drosera has acquired highly developed powers of movement, merely for the sake of bringing all its glands into contact with captured insects. So again, after a leaf of Dionaea has caught an insect, the slow pressing together of the two lobes serves merely to bring the glands on both sides into contact with it, causing also the secretion charged with animal matter to spread by capillary attraction over the whole surface. In the case of Pinguicula, as soon as an insect has been pushed for some little distance towards the midrib, immediate re-expansion would be beneficial, as the margins could not capture fresh prey until they were unfolded. The service rendered by this pushing action, as well as that from the marginal glands being brought into contact for a short time with the upper surfaces of minute captured insects, may perhaps account for the peculiar movements of the leaves; otherwise, we must look at these movements as a remnant of a more highly developed power formerly possessed by the progenitors of the genus.

In the four British species, and, as I hear from [page 380] Prof. Dyer, in most or all the species of the genus, the edges of the leaves are in some degree naturally and permanently incurved. This incurvation serves, as already shown, to prevent insects from being washed away by the rain; but it likewise serves for another end. When a number of glands have been powerfully excited by bits of meat, insects, or any other stimulus, the secretion often trickles down the leaf, and is caught by the incurved edges, instead of rolling off and being lost. As it runs down the channel, fresh glands are able to absorb the animal matter held in solution. Moreover, the secretion often collects in little pools within the channel, or in the spoon-like tips of the leaves; and I ascertained that bits of albumen, fibrin, and gluten, are here dissolved more quickly and completely than on the surface of the leaf, where the secretion cannot accumulate; and so it would be with naturally caught insects. The secretion was repeatedly seen thus to collect on the leaves of plants protected from the rain; and with exposed plants there would be still greater need of some provision to prevent, as far as possible, the secretion, with its dissolved animal matter, being wholly lost.

It has already been remarked that plants growing in a state of nature have the margins of their leaves much more strongly incurved than those grown in pots and prevented from catching many insects. We have seen that insects washed down by the rain from all parts of the leaf often lodge within the margins, which are thus excited to curl farther inwards; and we may suspect that this action, many times repeated during the life of the plant, leads to their permanent and well-marked incurvation. I regret that this view did not occur to me in time to test its truth.

It may here be added, though not immediately [page 381] bearing on our subject, that when a plant is pulled up, the leaves immediately curl downwards so as almost to conceal the roots,—a fact which has been noticed by many persons. I suppose that this is due to the same tendency which causes the outer and older leaves to lie flat on the ground. It further appears that the flower-stalks are to a certain extent irritable, for Dr. Johnson states that they "bend backwards if rudely handled."*

Secretion, Absorption, and Digestion.—I will first give my observations and experiments, and then a summary of the results.

[The Effects of Objects containing Soluble Nitrogenous Matter.

(1) Flies were placed on many leaves, and excited the glands to secrete copiously; the secretion always becoming acid, though not so before. After a time these insects were rendered so tender that their limbs and bodies could be separated by a mere touch, owing no doubt to the digestion and disintegration of their muscles. The glands in contact with a small fly continued to secrete for four days, and then became almost dry. A narrow strip of this leaf was cut off, and the glands of the longer and shorter hairs, which had lain in contact for the four days with the fly, and those which had not touched it, were compared under the microscope and presented a wonderful contrast. Those which had been in contact were filled with brownish granular matter, the others with homogeneous fluid. There could therefore be no doubt that the former had absorbed matter from the fly.

(2) Small bits of roast meat, placed on a leaf, always caused much acid secretion in the course of a few hours—in one case within 40 m. When thin fibres of meat were laid along the margin of a leaf which stood almost upright, the secretion ran down to the ground. Angular bits of meat, placed in little pools of the secretion near the margin, were in the course of

* 'English Botany,' by Sir J.E. Smith; with coloured figures by J. Sowerby; edit. of 1832, tab. 24, 25, 26. [page 382]

two or three days much reduced in size, rounded, rendered more or less colourless and transparent, and so much softened that they fell to pieces on the slightest touch. In only one instance was a very minute particle completely dissolved, and this occurred within 48 hrs. When only a small amount of secretion was excited, this was generally absorbed in from 24 hrs. to 48 hrs.; the glands being left dry. But when the supply of secretion was copious, round either a single rather large bit of meat, or round several small bits, the glands did not become dry until six or seven days had elapsed. The most rapid case of absorption observed by me was when a small drop of an infusion of raw meat was placed on a leaf, for the glands here became almost dry in 3 hrs. 20 m. Glands excited by small particles of meat, and which have quickly absorbed their own secretion, begin to secrete again in the course of seven or eight days from the time when the meat was given them.

(3) Three minute cubes of tough cartilage from the leg-bone of a sheep were laid on a leaf. After 10 hrs. 30 m. some acid secretion was excited, but the cartilage appeared little or not at all affected. After 24 hrs. the cubes were rounded and much reduced in size; after 32 hrs. they were softened to the centre, and one was quite liquefied; after 35 hrs. mere traces of solid cartilage were left; and after 48 hrs. a trace could still be seen through a lens in only one of the three. After 82 hrs. not only were all three cubes completely liquefied, but all the secretion was absorbed and the glands left dry.

(4) Small cubes of albumen were placed on a leaf; in 8 hrs. feebly acid secretion extended to a distance of nearly 1/10 of an inch round them, and the angles of one cube were rounded. After 24 hrs. the angles of all the cubes were rounded, and they were rendered throughout very tender; after 30 hrs. the secretion began to decrease, and after 48 hrs. the glands were left dry; but very minute bits of albumen were still left undissolved.

(5) Smaller cubes of albumen (about 1/50 or 1/60 of an inch, .508 or .423 mm.) were placed on four glands; after 18 hrs. one cube was completely dissolved, the others being much reduced in size, softened, and transparent. After 24 hrs. two of the cubes were completely dissolved, and already the secretion on these glands was almost wholly absorbed. After 42 hrs. the two other cubes were completely dissolved. These four glands began to secrete again after eight or nine days.

(6) Two large cubes of albumen (fully 1/20 of an inch, 1.27 mm.) were placed, one near the midrib and the other near the margin [page 383] of a leaf; in 6 hrs. there was much secretion, which after 48 hrs. accumulated in a little pool round the cube near the margin. This cube was much more dissolved than that on the blade of the leaf; so that after three days it was greatly reduced in size, with all the angles rounded, but it was too large to be wholly dissolved. The secretion was partially absorbed after four days. The cube on the blade was much less reduced, and the glands on which it rested began to dry after only two days.

(7) Fibrin excites less secretion than does meat or albumen. Several trials were made, but I will give only three of them. Two minute shreds were placed on some glands, and in 3 hrs. 45 m. their secretion was plainly increased. The smaller shred of the two was completely liquefied in 6 hrs. 15 m., and the other in 24 hrs.; but even after 48 hrs. a few granules of fibrin could still be seen through a lens floating in both drops of secretion. After 56 hrs. 30 m. these granules were completely dissolved. A third shred was placed in a little pool of secretion, within the margin of a leaf where a seed had been lying, and this was completely dissolved in the course of 15 hrs. 30 m.

(8) Five very small bits of gluten were placed on a leaf, and they excited so much secretion that one of the bits glided down into the marginal furrow. After a day all five bits seemed much reduced in size, but none were wholly dissolved. On the third day I pushed two of them, which had begun to dry, on to fresh glands. On the fourth day undissolved traces of three out of the five bits could still be detected, the other two having quite disappeared; but I am doubtful whether they had really been completely dissolved. Two fresh bits were now placed, one near the middle and the other near the margin of another leaf; both excited an extraordinary amount of secretion; that near the margin had a little pool formed round it, and was much more reduced in size than that on the blade, but after four days was not completely dissolved. Gluten, therefore, excites the glands greatly, but is dissolved with much difficulty, exactly as in the case of Drosera. I regret that I did not try this substance after having been immersed in weak hydrochloric acid, as it would then probably have been quickly dissolved.

(9) A small square thin piece of pure gelatine, moistened with water, was placed on a leaf, and excited very little secretion in 5 hrs. 30 m., but later in the day a greater amount. After 24 hrs. the whole square was completely liquefied; and this would not have occurred had it been left in water. The liquid was acid.

(10) Small particles of chemically prepared casein excited [page 384] acid secretion, but were not quite dissolved after two days; and the glands then began to dry. Nor could their complete dissolution have been expected from what we have seen with Drosera.

(11) Minute drops of skimmed milk were placed on a leaf, and these caused the glands to secrete freely. After 3 hrs. the milk was found curdled, and after 23 hrs. the curds were dissolved. On placing the now clear drops under the microscope, nothing could be detected except some oil-globules. The secretion, therefore, dissolves fresh casein.

(12) Two fragments of a leaf were immersed for 17 hrs., each in a drachm of a solution of carbonate of ammonia, of two strengths, namely of one part to 437 and 218 of water. The glands of the longer and shorter hairs were then examined, and their contents found aggregated into granular matter of a brownish-green colour. These granular masses were seen by my son slowly to change their forms, and no doubt consisted of protoplasm. The aggregation was more strongly pronounced, and the movements of the protoplasm more rapid, within the glands subjected to the stronger solution than in the others. The experiment was repeated with the same result; and on this occasion I observed that the protoplasm had shrunk a little from the walls of the single elongated cells forming the pedicels. In order to observe the process of aggregation, a narrow strip of leaf was laid edgeways under the microscope, and the glands were seen to be quite transparent; a little of the stronger solution (viz. one part to 218 of water) was now added under the covering glass; after an hour or two the glands contained very fine granular matter, which slowly became coarsely granular and slightly opaque; but even after 5 hrs. not as yet of a brownish tint. By this time a few rather large, transparent, globular masses appeared within the upper ends of the pedicels, and the protoplasm lining their walls had shrunk a little. It is thus evident that the glands of Pinguicula absorb carbonate of ammonia; but they do not absorb it, or are not acted on by it, nearly so quickly as those of Drosera.

(13) Little masses of the orange-coloured pollen of the common pea, placed on several leaves, excited the glands to secrete freely. Even a very few grains which accidentally fell on a single gland caused the drop surrounding it to increase so much in size, in 23 hrs., as to be manifestly larger than the drops on the adjoining glands. Grains subjected to the secretion for 48 hrs. did not emit their tubes; they were quite discoloured, and seemed to contain less matter than before; that [page 385] which was left being of a dirty colour, including globules of oil. They thus differed in appearance from other grains kept in water for the same length of time. The glands in contact with the pollen-grains had evidently absorbed matter from them; for they had lost their natural pale-green tint, and contained aggregated globular masses of protoplasm.

(14) Square bits of the leaves of spinach, cabbage, and a saxifrage, and the entire leaves of Erica tetralix, all excited the glands to increased secretion. The spinach was the most effective, for it caused the secretion evidently to increase in 1 hr. 40 m., and ultimately to run some way down the leaf; but the glands soon began to dry, viz. after 35 hrs. The leaves of Erica tetralix began to act in 7 hrs. 30 m., but never caused much secretion; nor did the bits of leaf of the saxifrage, though in this case the glands continued to secrete for seven days. Some leaves of Pinguicula were sent me from North Wales, to which leaves of Erica tetralixand of an unknown plant adhered; and the glands in contact with them had their contents plainly aggregated, as if they had been in contact with insects; whilst the other glands on the same leaves contained only clear homogeneous fluid.

(15) Seeds.—A considerable number of seeds or fruits selected by hazard, some fresh and some a year old, some soaked for a short time in water and some not soaked, were tried. The ten following kinds, namely cabbage, radish, Anemone nemorosa, Rumex acetosa, Carex sylvatica, mustard, turnip, cress, Ranunculus acris, and Avena pubescens, all excited much secretion, which was in several cases tested and found always acid. The five first-named seeds excited the glands more than the others. The secretion was seldom copious until about 24 hrs. had elapsed, no doubt owing to the coats of the seeds not being easily permeable. Nevertheless, cabbage seeds excited some secretion in 4 hrs. 30 m.; and this increased so much in 18 hrs. as to run down the leaves. The seeds or properly the fruits of Carex are much oftener found adhering to leaves in a state of nature than those of any other genus; and the fruits of Carex sylvatica excited so much secretion that in 15 hrs. it ran into the incurved edges; but the glands ceased to secrete after 40 hrs. On the other hand, the glands on which the seeds of the Rumex and Avena rested continued to secrete for nine days.

The nine following kinds of seeds excited only a slight amount of secretion, namely, celery, parsnip, caraway, Linum grandiflorum, Cassia, Trifolium pannonicum, Plantago, onion, [page 386] and Bromus. Most of these seeds did not excite any secretion until 48 hrs. had elapsed, and in the case of the Trifolium only one seed acted, and this not until the third day. Although the seeds of the Plantago excited very little secretion, the glands continued to secrete for six days. Lastly, the five following kinds excited no secretion, though left on the leaves for two or three days, namely lettuce, Erica tetralix, Atriplex hortensis, Phalaris canariensis, and wheat. Nevertheless, when the seeds of the lettuce, wheat, and Atriplex were split open and applied to leaves, secretion was excited in considerable quantity in 10 hrs., and I believe that some was excited in six hours. In the case of the Atriplex the secretion ran down to the margin, and after 24 hrs. I speak of it in my notes "as immense in quantity and acid." The split seeds also of the Trifolium and celery acted powerfully and quickly, though the whole seeds caused, as we have seen, very little secretion, and only after a long interval of time. A slice of the common pea, which however was not tried whole, caused secretion in 2 hrs. From these facts we may conclude that the great difference in the degree and rate at which various kinds of seeds excite secretion, is chiefly or wholly due to the different permeability of their coats.

Some thin slices of the common pea, which had been previously soaked for 1 hr. in water, were placed on a leaf, and quickly excited much acid secretion. After 24 hrs. these slices were compared under a high power with others left in water for the same time; the latter contained so many fine granules of legumin that the slide was rendered muddy; whereas the slices which had been subjected to the secretion were much cleaner and more transparent, the granules of legumin apparently having been dissolved. A cabbage seed which had lain for two days on a leaf and had excited much acid secretion, was cut into slices, and these were compared with those of a seed which had been left for the same time in water. Those subjected to the secretion were of a paler colour; their coats presenting the greatest differences, for they were of a pale dirty tint instead of chestnut-brown. The glands on which the cabbage seeds had rested, as well as those bathed by the surrounding secretion, differed greatly in appearance from the other glands on the same leaf, for they all contained brownish granular matter, proving that they had absorbed matter from the seeds.

That the secretion acts on the seeds was also shown by some of them being killed, or by the seedlings being injured. Fourteen cabbage seeds were left for three days on leaves and excited [page 387] much secretion; they were then placed on damp sand under conditions known to be favourable for germination. Three never germinated, and this was a far larger proportion of deaths than occurred with seeds of the same lot, which had not been subjected to the secretion, but were otherwise treated in the same manner. Of the eleven seedlings raised, three had the edges of their cotyledons slightly browned, as if scorched; and the cotyledons of one grew into a curious indented shape. Two mustard seeds germinated; but their cotyledons were marked with brown patches and their radicles deformed. Of two radish seeds, neither germinated; whereas of many seeds of the same lot not subjected to the secretion, all, excepting one, germinated. Of the two Rumex seeds, one died and the other germinated; but its radicle was brown and soon withered. Both seeds of the Avena germinated, one grew well, the other had its radicle brown and withered. Of six seeds of the Erica none germinated, and when cut open after having been left for five months on damp sand, one alone seemed alive. Twenty-two seeds of various kinds were found adhering to the leaves of plants growing in a state of nature; and of these, though kept for five months on damp sand, none germinated, some being then evidently dead.

The Effects of Objects not containing Soluble Nitrogenous Matter.

(16) It has already been shown that bits of glass, placed on leaves, excite little or no secretion. The small amount which lay beneath the fragments was tested and found not acid. A bit of wood excited no secretion; nor did the several kinds of seeds of which the coats are not permeable to the secretion, and which, therefore, acted like inorganic bodies. Cubes of fat, left for two days on a leaf, produced no effect.

(17) A particle of white sugar, placed on a leaf, formed in 1 hr. 10 m. a large drop of fluid, which in the course of 2 additional hours ran down into the naturally inflected margin. This fluid was not in the least acid, and began to dry up, or more probably was absorbed, in 5 hrs. 30 m. The experiment was repeated; particles being placed on a leaf, and others of the same size on a slip of glass in a moistened state; both being covered by a bell-glass. This was done to see whether the increased amount of fluid on the leaves could be due to mere deliquescence; but this was proved not to be the case. The particle on the leaf caused so much secretion that in the course of 4 hrs. it ran down across two-thirds of the leaf. After 8 hrs. the leaf, which was concave, was actually filled with very viscid [page 388] fluid; and it particularly deserves notice that this, as on the former occasion, was not in the least acid. This great amount of secretion may be attributed to exosmose. The glands which had been covered for 24 hrs. by this fluid did not differ, when examined under the microscope, from others on the same leaf, which had not come into contact with it. This is an interesting fact in contrast with the invariably aggregated condition of glands which have been bathed by the secretion, when holding animal matter in solution.

(18) Two particles of gum arabic were placed on a leaf, and they certainly caused in 1 hr. 20 m. a slight increase of secretion. This continued to increase for the next 5 hrs., that is for as long a time as the leaf was observed.

(19) Six small particles of dry starch of commerce were placed on a leaf, and one of these caused some secretion in 1 hr. 15 m., and the others in from 8 hrs. to 9 hrs. The glands which had thus been excited to secrete soon became dry, and did not begin to secrete again until the sixth day. A larger bit of starch was then placed on a leaf, and no secretion was excited in 5 hrs. 30 m.; but after 8 hrs. there was a considerable supply, which increased so much in 24 hrs. as to run down the leaf to the distance of 3/4 of an inch. This secretion, though so abundant, was not in the least acid. As it was so copiously excited, and as seeds not rarely adhere to the leaves of naturally growing plants, it occurred to me that the glands might perhaps have the power of secreting a ferment, like ptyaline, capable of dissolving starch; so I carefully observed the above six small particles during several days, but they did not seem in the least reduced in bulk. A particle was also left for two days in a little pool of secretion, which had run down from a piece of spinach leaf; but although the particle was so minute no diminution was perceptible. We may therefore conclude that the secretion cannot dissolve starch. The increase caused by this substance may, I presume, be attributed to exosmose. But I am surprised that starch acted so quickly and powerfully as it did, though in a less degree than sugar. Colloids are known to possess some slight power of dialysis; and on placing the leaves of a Primula in water, and others in syrup and diffused starch, those in the starch became flaccid, but to a less degree and at a much slower rate than the leaves in the syrup; those in water remaining all the time crisp.]

From the foregoing experiments and observations we [page 389] see that objects not containing soluble matter have little or no power of exciting the glands to secrete. Non-nitrogenous fluids, if dense, cause the glands to pour forth a large supply of viscid fluid, but this is not in the least acid. On the other hand, the secretion from glands excited by contact with nitrogenous solids or liquids is invariably acid, and is so copious that it often runs down the leaves and collects within the naturally incurved margins. The secretion in this state has the power of quickly dissolving, that is of digesting, the muscles of insects, meat, cartilage, albumen, fibrin, gelatine, and casein as it exists in the curds of milk. The glands are strongly excited by chemically prepared casein and gluten; but these substances (the latter not having been soaked in weak hydrochloric acid) are only partially dissolved, as was likewise the case with Drosera. The secretion, when containing animal matter in solution, whether derived from solids or from liquids, such as an infusion of raw meat, milk, or a weak solution of carbonate of ammonia, is quickly absorbed; and the glands, which were before limpid and of a greenish colour, become brownish and contain masses of aggregated granular matter. This matter, from its spontaneous movements, no doubt consists of protoplasm. No such effect is produced by the action of non-nitrogenous fluids. After the glands have been excited to secrete freely, they cease for a time to secrete, but begin again in the course of a few days.

Glands in contact with pollen, the leaves of other plants, and various kinds of seeds, pour forth much acid secretion, and afterwards absorb matter probably of an albuminous nature from them. Nor can the benefit thus derived be insignificant, for a considerable [page 390] amount of pollen must be blown from the many wind-fertilised carices, grasses, &c., growing where Pinguicula lives, on to the leaves thickly covered with viscid glands and forming large rosettes. Even a few grains of pollen on a single gland causes it to secrete copiously. We have also seen how frequently the small leaves of Erica tetralix and of other plants, as well as various kinds of seeds and fruits, especially of Carex, adhere to the leaves. One leaf of the Pinguicula had caught ten of the little leaves of the Erica; and three leaves on the same plant had each caught a seed. Seeds subjected to the action of the secretion are sometimes killed, or the seedlings injured. We may, therefore, conclude that Pinguicula vulgaris, with its small roots, is not only supported to a large extent by the extraordinary number of insects which it habitually captures, but likewise draws some nourishment from the pollen, leaves, and seeds of other plants which often adhere to its leaves. It is therefore partly a vegetable as well as an animal feeder.

PINGUICULA GRANDIFLORA.

This species is so closely allied to the last that it is ranked by Dr. Hooker as a sub-species. It differs chiefly in the larger size of its leaves, and in the glandular hairs near the basal part of the midrib being longer. But it likewise differs in constitution; I hear from Mr. Ralfs, who was so kind as to send me plants from Cornwall, that it grows in rather different sites; and Dr. Moore, of the Glasnevin Botanic Gardens, informs me that it is much more manageable under culture, growing freely and flowering annually; whilst Pinguicula vulgaris has to be renewed every year. Mr. Ralfs found numerous [page 391] insects and fragments of insects adhering to almost all the leaves. These consisted chiefly of Diptera, with some Hymenoptera, Homoptera, Coleoptera, and a moth. On one leaf there were nine dead insects, besides a few still alive. He also observed a few fruits of Carex pulicaris, as well as the seeds of this same Pinguicula, adhering to the leaves. I tried only two experiments with this species; firstly, a fly was placed near the margin of a leaf, and after 16 hrs. this was found well inflected. Secondly, several small flies were placed in a row along one margin of another leaf, and by the next morning this whole margin was curled inwards, exactly as in the case of Pinguicula vulgaris.

PINGUICULA LUSITANICA.

This species, of which living specimens were sent me by Mr. Ralfs from Cornwall, is very distinct from the two foregoing ones. The leaves are rather smaller, much more transparent, and are marked with purple branching veins. The margins of the leaves are much more involuted; those of the older ones extending over a third of the space between the midrib and the outside. As in the two other species, the glandular hairs consist of longer and shorter ones, and have the same structure; but the glands differ in being purple, and in often containing granular matter before they have been excited. In the lower part of the leaf, almost half the space on each side between the midrib and margin is destitute of glands; these being replaced by long, rather stiff, multicellular hairs, which intercross over the midrib. These hairs perhaps serve to prevent insects from settling on this part of the leaf, where there are no viscid glands by which they could be caught; but it is hardly probable that they were developed for this purpose. The spiral vessels pro- [page 392] ceeding from the midrib terminate at the extreme margin of the leaf in spiral cells; but these are not so well developed as in the two preceding species. The flower-peduncles, sepals, and petals, are studded with glandular hairs, like those on the leaves.

The leaves catch many small insects, which are found chiefly beneath the involuted margins, probably washed there by the rain. The colour of the glands on which insects have long lain is changed, being either brownish or pale purple, with their contents coarsely granular; so that they evidently absorb matter from their prey. Leaves of the Erica tetralix, flowers of a Galium, scales of grasses, &c. likewise adhered to some of the leaves. Several of the experiments which were tried on Pinguicula vulgaris were repeated on Pinguicula lusitanica, and these will now be given.

[(1) A moderately sized and angular bit of albumen was placed on one side of a leaf, halfway between the midrib and the naturally involuted margin. In 2 hrs. 15 m. the glands poured forth much secretion, and this side became more infolded than the opposite one. The inflection increased, and in 3 hrs. 30 m. extended up almost to the apex. After 24 hrs. the margin was rolled into a cylinder, the outer surface of which touched the blade of the leaf and reached to within the 1/20 of an inch of the midrib. After 48 hrs. it began to unfold, and in 72 hrs. was completely unfolded. The cube was rounded and greatly reduced in size; the remainder being in a semi-liquefied state.

(2) A moderately sized bit of albumen was placed near the apex of a leaf, under the naturally incurved margin. In 2 hrs. 30 m. much secretion was excited, and next morning the margin on this side was more incurved than the opposite one, but not to so great a degree as in the last case. The margin unfolded at the same rate as before. A large proportion of the albumen was dissolved, a remnant being still left.

(3) Large bits of albumen were laid in a row on the midribs of two leaves, but produced in the course of 24 hrs. no effect; [page 393] nor could this have been expected, for even had glands existed here, the long bristles would have prevented the albumen from coming in contact with them. On both leaves the bits were now pushed close to one margin, and in 3 hrs. 30 m. this became so greatly inflected that the outer surface touched the blade; the opposite margin not being in the least affected. After three days the margins of both leaves with the albumen were still as much inflected as ever, and the glands were still secreting copiously. With Pinguicula vulgaris I have never seen inflection lasting so long.

(4) Two cabbage seeds, after being soaked for an hour in water, were placed near the margin of a leaf, and caused in 3 hrs. 20 m. increased secretion and incurvation. After 24 hrs. the leaf was partially unfolded, but the glands were still secreting freely. These began to dry in 48 hrs., and after 72 hrs. were almost dry. The two seeds were then placed on damp sand under favourable conditions for growth; but they never germinated, and after a time were found rotten. They had no doubt been killed by the secretion.

(5) Small bits of a spinach leaf caused in 1 hr. 20 m. increased secretion; and after 3 hrs. 20 m. plain incurvation of the margin. The margin was well inflected after 9 hrs. 15 m., but after 24 hrs. was almost fully re-expanded. The glands in contact with the spinach became dry in 72 hrs. Bits of albumen had been placed the day before on the opposite margin of this same leaf, as well as on that of a leaf with cabbage seeds, and these margins remained closely inflected for 72 hrs., showing how much more enduring is the effect of albumen than of spinach leaves or cabbage seeds .

(6) A row of small fragments of glass was laid along one margin of a leaf; no effect was produced in 2 hrs. 10 m., but after 3 hrs. 25 m. there seemed to be a trace of inflection, and this was distinct, though not strongly marked, after 6 hrs. The glands in contact with the fragments now secreted more freely than before; so that they appear to be more easily excited by the pressure of inorganic objects than are the glands of Pinguicula vulgaris. The above slight inflection of the margin had not increased after 24 hrs., and the glands were now beginning to dry. The surface of a leaf, near the midrib and towards the base, was rubbed and scratched for some time, but no movement ensued. The long hairs which are situated here were treated in the same manner, with no effect. This latter trial was made because I thought that the hairs might perhaps be sensitive to a touch, like the filaments of Dionaea. [page 394]