(24.) A rather large oblong piece of card fixed with shellac to apex: after 24 h. no effect, but the card was found not to be touching the apex. A small square was now refixed with shellac; after 16 h. slight deflection from the perpendicular and from the card. After an additional day the radicle became almost straight.

(25.) Square of card fixed laterally to apex of young radicle; after 9 h. deflection from the perpendicular considerable; after 24 h. deflection reduced. Refixed a fresh square with shellac: after 24 h. deflection about 40o from the perpendicular and from the card.

(26.) A very small square of card fixed with shellac to apex of young radicle: after 9 h. the deflection from the perpendicular and from the card amounted to nearly a right angle; after 24 h. deflection much reduced; after an additional 24 h. radicle almost straight.

(27.) Square of card fixed with shellac to apex of young radicle: after 9 h. deflection from the card and from the perpendicular a right angle; next morning quite straight. Refixed a square laterally with shellac; after 9 h. a little deflection, which after 24 h. increased to nearly 20o from the perpendicular and from the card.

(28.) Square of card fixed with shellac; after 9 h. some deflection; next morning the card dropped off; refixed it with shellac; it again became loose and was refixed; and now on the third trial the radicle was deflected after 14 h. at right angles from the card.

(29.) A small square of card was first fixed with thick gum-water to the apex. It produced a slight effect but soon fell off. A similar square was now affixed laterally with shellac: after 9 h. the radicle was deflected nearly 45o from the perpendicular and from the card. After 36 additional hours angle of deflection reduced to about 30o.

(30.) A very small piece, less than 1/20th of an inch square, of thin tin-foil fixed with shellac to the apex of a young radicle; after 24 h. no effect. Tin-foil removed, and a small square of sanded card fixed with shellac; after 9 h. deflection at nearly right angles from the perpendicular and from the card. Next [page 139] morning deflection reduced to about 40o from the perpendicular.

(31.) A splinter of thin glass gummed to apex, after 9 h. no effect, but it was then found not to be touching the apex of the radicle. Next morning a square of card was fixed with shellac to it, and after 9 h. radicle greatly deflected from the card. After two additional days the deflection had decreased and was only 35o from the perpendicular.

(32.) Small square of sanded card, attached with thick gum-water laterally to the apex of a long straight radicle: after 9 h. greatly deflected from the perpendicular and from the card. Curvature extended for a length of .22 of an inch from the apex. After 3 additional hours terminal portion deflected at right angles from the perpendicular. Next morning the curved portion was .36 in length.

(33.) Square of card gummed to apex: after 15 h. deflected at nearly 90o from the perpendicular and from the card.

(34.) Small oblong of sanded card gummed to apex: after 15 h. deflected 90o from the perpendicular and from the card: in the course of the three following days the terminal portion became much contorted and ultimately coiled into a helix.

(35.) Square of card gummed to apex: after 9 h. deflected from card: after 24 h. from time of attachment greatly deflected obliquely and partly in opposition to Sachs' curvature.

(36.) Small piece of card, rather less than 1/20th of an inch square, gummed to apex: in 9 h. considerably deflected from card and in opposition to Sachs' curvature; after 24 h. greatly deflected in the same direction. After an additional day the extreme tip was curved towards the card.

(37.) Square of card, gummed to apex in front, caused after 8 h. 30 m. hardly any effect; refixed fresh square laterally, after 15 h. deflected almost 90o from the perpendicular and from the card. After 2 additional days deflection much reduced.

(38.) Square of card gummed to apex: after 9 h. much deflection, which after 24 h. from time of fixing increased to nearly 90o. After an additional day terminal portion was curled into a loop, and on the following day into a helix.

(39.) Small oblong piece of card gummed to apex, nearly in front, but a little to one side; in 9 h. slightly deflected in the direction of Sachs' curvature, but rather obliquely, and to side opposite to card. Next day more curved in the same direction, and after 2 additional days coiled into a ring. [page 140]

(40.) Square of card gummed to apex: after 9 h. slightly curved from card; next morning radicle straight, and apex had grown beyond the card. Refixed another square laterally with shellac; in 9 h. deflected laterally, but also in the direction of Sachs' curvature. After 2 additional days' curvature considerably increased in the same direction.

(41.) Little square of tin-foil fixed with gum to one side of apex of a young and short radicle: after 15 h. no effect, but tin-foil had become displaced. A little square of card was now gummed to one side of apex, which after 8 h. 40 m. was slightly deflected; in 24 h. from the time of attachment deflected at 90o from the perpendicular and from the card; after 9 additional hours became hooked, with the apex pointing to the zenith. In 3 days from the time of attachment the terminal portion of the radicle formed a ring or circle.

(42.) A little square of thick letter-paper gummed to the apex of a radicle, which after 9 h. was deflected from it. In 24 h. from time when the paper was affixed the deflection much increased, and after 2 additional days it amounted to 50o from the perpendicular and from the paper.

(43.) A narrow chip of a quill was fixed with shellac to the apex of a radicle. After 9 h. no effect; after 24 h. moderate deflection, but now the quill had ceased to touch the apex. Removed quill and gummed a little square of card to apex, which after 8 h. caused slight deflection. On the fourth day from the first attachment of any object, the extreme tip was curved towards the card.

(44.) A rather long and narrow splinter of extremely thin glass, fixed with shellac to apex, it caused in 9 h. slight deflection, which disappeared in 24 h.; the splinter was then found not touching the apex. It was twice refixed, with nearly similar results, that is, it caused slight deflection, which soon disappeared. On the fourth day from the time of first attachment the tip was bent towards the splinter.]

From these experiments it is clear that the apex of the radicle of the bean is sensitive to contact, and that it causes the upper part to bend away from the touching object. But before giving a summary of the results, it will be convenient briefly to give a few other observations. Bits of very thin glass and little squares [page 141] of common card were affixed with thick gum-water to the tips of the radicles of seven beans, as a preliminary trial. Six of these were plainly acted on, and in two cases the radicles became coiled up into complete loops. One radicle was curved into a semi-circle in so short a period as 6 h. 10 m. The seventh radicle which was not affected was apparently sickly, as it became brown on the following day; so that it formed no real exception. Some of these trials were made in the early spring during cold weather in a sitting-room, and others in a greenhouse, but the temperature was not recorded. These six striking cases almost convinced us that the apex was sensitive, but of course we determined to make many more trials. As we had noticed that the radicles grew much more quickly when subjected to considerable heat, and as we imagined that heat would increase their sensitiveness, vessels with germinating beans suspended in damp air were placed on a chimney-piece, where they were subjected during the greater part of the day to a temperature of between 69o and 72o F.; some, however, were placed in the hot-house where the temperature was rather higher. Above two dozen beans were thus tried; and when a square of glass or card did not act, it was removed, and a fresh one affixed, this being often done thrice to the same radicle. Therefore between five and six dozen trials were altogether made. But there was moderately distinct deflection from the perpendicular and from the attached object in only one radicle out of this large number of cases. In five other cases there was very slight and doubtful deflection. We were astonished at this result, and concluded that we had made some inexplicable mistake in the first six experiments. But before finally relinquishing the subject, we resolved to make one [page 142] other trial for it occurred to us that sensitiveness is easily affected by external conditions, and that radicles growing naturally in the earth in the early spring would not be subjected to a temperature nearly so high as 70o F. We therefore allowed the radicles of 12 beans to grow at a temperature of between 55o and 60o F. The result was that in every one of these cases (included in the above-described experiments) the radicle was deflected in the course of a few hours from the attached object. All the above recorded successful trials, and some others presently to be given, were made in a sitting-room at the temperatures just specified. It therefore appears that a temperature of about, or rather above, 70o F. destroys the sensitiveness of the radicles, either directly, or indirectly through abnormally accelerated growth; and this curious fact probably explains why Sachs, who expressly states that his beans were kept at a high temperature, failed to detect the sensitiveness of the apex of the radicle.

But other causes interfere with this sensibility. Eighteen radicles were tried with little squares of sanded card, some affixed with shellac and some with gum-water, during the few last days of 1878, and few first days of the next year. They were kept in a room at the proper temperature during the day, but were probably too cold at night, as there was a hard frost at the time. The radicles looked healthy but grew very slowly. The result was that only 6 out of the 18 were deflected from the attached cards, and this only to a slight degree and at a very slow rate. These radicles therefore presented a striking contrast with the 44 above described. On March 6th and 7th, when the temperature of the room varied between 53o and 59o F., eleven germinating beans were tried in the [page 143] same manner, and now every one of the radicles became curved away from the cards, though one was only slightly deflected. Some horticulturists believe that certain kinds of seeds will not germinate properly in the middle of the winter, although kept at a right temperature. If there really is any proper period for the germination of the bean, the feeble degree of sensibility of the above radicles may have resulted from the trial having been made in the middle of the winter, and not simply from the nights being too cold. Lastly, the radicles of four beans, which from some innate cause germinated later than all the others of the same lot, and which grew slowly though appearing healthy, were similarly tried, and even after 24 h. they were hardly at all deflected from the attached cards. We may therefore infer that any cause which renders the growth of the radicles either slower or more rapid than the normal rate, lessens or annuls the sensibility of their tips to contact. It deserves particular attention that when the attached objects failed to act, there was no bending of any kind, excepting Sachs' curvature. The force of our evidence would have been greatly weakened if occasionally, though rarely, the radicles had become curved in any direction independently of the attached objects. In the foregoing numbered paragraphs, however, it may be observed that the extreme tip sometimes becomes, after a considerable interval of time, abruptly curved towards the bit of card; but this is a totally distinct phenomenon, as will presently be explained.

Summary of the Results of the foregoing Experiments on the Radicles of Vicia faba.—Altogether little squares (about 1/20th of an inch), generally of sanded paper as stiff as thin card (between .15 and .20 mm. in thickness), sometimes of ordinary card, or little frag- [page 144] ments of very thin glass etc., were affixed at different times to one side of the conical tips of 55 radicles. The 11 last-mentioned cases, but not the preliminary ones, are here included. The squares, etc., were most commonly affixed with shellac, but in 19 cases with thick gum-water. When the latter was used, the squares were sometimes found, as previously stated, to be separated from the apex by a layer of thick fluid, so that there was no contact, and consequently no bending of the radicle; and such few cases were not recorded. But in every instance in which shellac was employed, unless the square fell off very soon, the result was recorded. In several instances when the squares became displaced, so as to stand parallel to the radicle, or were separated by fluid from the apex, or soon fell off, fresh squares were attached, and these cases (described under the numbered paragraphs) are here included. Out of 55 radicles experimented on under the proper temperature, 52 became bent, generally to a considerable extent from the perpendicular, and away from the side to which the object was attached. Of the three failures, one can be accounted for, as the radicle became sickly on the following day; and a second was observed only during 11 h. 30 m. As in several cases the terminal growing part of the radicle continued for some time to bend from the attached object, it formed itself into a hook, with the apex pointing to the zenith, or even into a ring, and occasionally into a spire or helix. It is remarkable that these latter cases occurred more frequently when objects were attached with thick gum-water, which never became dry, than when shellac was employed. The curvature was often well-marked in from 7 h. to 11 h.; and in one instance a semicircle was formed in 6 h. 10 m, from the time [page 145] of attachment. But in order to see the phenomenon as well displayed as in the above described cases, it is indispensable that the bits of card, etc., should be made to adhere closely to one side of the conical apex; that healthy radicles should be selected and kept at not too high or too low a temperature, and apparently that the trials should not be made in the middle of the winter.

In ten instances, radicles which had curved away from a square of card or other object attached to their tips, straightened themselves to a certain extent, or even completely, in the course of from one to two days from the time of attachment. This was more especially apt to occur when the curvature was slight. But in one instance (No. 27) a radicle which in 9 h. had been deflected about 90o from the perpendicular, became quite straight in 24 h. from the period of attachment. With No. 26, the radicle was almost straight in 48 h. We at first attributed the straightening process to the radicles becoming accustomed to a slight stimulus, in the same manner as a tendril or sensitive petiole becomes accustomed to a very light loop of thread, and unbends itself though the loop remains still suspended; but Sachs states* that radicles of the bean placed horizontally in damp air after curving downwards through geotropism, straighten themselves a little by growth along their lower or concave sides. Why this should occur is not clear: but perhaps it likewise occurred in the above ten cases. There is another occasional movement which must not be passed over: the tip of the radicle, for a length of from 2 to 3 mm., was found in six instances,

* 'Arbeiten Bot. Instit., Wrzburg,' Heft iii. p. 456. [page 146]

after an interval of about 24 or more hours, bent towards the bit of still attached card,—that is, in a direction exactly opposite to the previously induced curvature of the whole growing part for a length of from 7 to 8 mm. This occurred chiefly when the first curvature was small, and when an object had been affixed more than once to the apex of the same radicle. The attachment of a bit of card by shellac to one side of the tender apex may sometimes mechanically prevent its growth; or the application of thick gum-water more than once to the same side may injure it; and then checked growth on this side with continued growth on the opposite and unaffected side would account for the reversed curvature of the apex.

Various trials were made for ascertaining, as far as we could, the nature and degree of irritation to which the apex must be subjected, in order that the terminal growing part should bend away, as if to avoid the cause of irritation. We have seen in the numbered experiments, that a little square of rather thick letter-paper gummed to the apex induced, though slowly, considerable deflection. Judging from several cases in which various objects had been affixed with gum, and had soon become separated from the apex by a layer of fluid, as well as from some trials in which drops of thick gum-water alone had been applied, this fluid never causes bending. We have also seen in the numbered experiments that narrow splinters of quill and of very thin glass, affixed with shellac, caused only a slight degree of deflection, and this may perhaps have been due to the shellac itself. Little squares of goldbeaters' skin, which is excessively thin, were damped, and thus made to adhere to one side of the tips of two radicles; one of these, after 24 h., produced no effect; nor did the [page 147] other in 8 h., within which time squares of card usually act; but after 24 h. there was slight deflection.

An oval bead, or rather cake, of dried shellac, 1.01 mm. in length and 0.63 in breadth, caused a radicle to become deflected at nearly right angles in the course of only 6 h.; but after 23 h. it had nearly straightened itself. A very small quantity of dissolved shellac was spread over a bit of card, and the tips of 9 radicles were touched laterally with it; only two of them became slightly deflected to the side opposite to that bearing the speck of dried shellac, and they afterwards straightened themselves. These specks were removed, and both together weighed less than 1/100th of a grain; so that a weight of rather less than 1/200th of a grain (0.32 mg.) sufficed to excite movement in two out of the nine radicles. Here then we have apparently reached nearly the minimum weight which will act.

A moderately thick bristle (which on measurement was found rather flattened, being 0.33 mm. in one diameter, and 0.20 mm. in the other) was cut into lengths of about 1/20th of an inch. These after being touched with thick gum-water, were placed on the tips of eleven radicles. Three of them were affected; one being deflected in 8 h. 15 m. to an angle of about 90o from the perpendicular; a second to the same amount when looked at after 9 h.; but after 24 h. from the time of first attachment the deflection had decreased to only 19o; the third was only slightly deflected after 9 h., and the bit of bristle was then found not touching the apex; it was replaced, and after 15 additional hours the deflection amounted to 26o from the perpendicular. The remaining eight radicles were not at all acted on by the bits of bristle, so that we here appear to have nearly reached the minimum [page 148] of size of an object which will act on the radicle of the bean. But it is remarkable that when the bits of bristle did act, that they should have acted so quickly and efficiently.

As the apex of a radicle in penetrating the ground must be pressed on all sides, we wished to learn whether it could distinguish between harder or more resisting, and softer substances. A square of the sanded paper, almost as stiff as card, and a square of extremely thin paper (too thin for writing on), of exactly the same size (about 1/20th of an inch), were fixed with shellac on opposite sides of the apices of 12 suspended radicles. The sanded card was between 0.15 and 0.20 mm. (or between 0.0059 and 0.0079 of an inch), and the thin paper only 0.045 mm. (or 0.00176 of an inch) in thickness. In 8 out of the 12 cases there could be no doubt that the radicle was deflected from the side to which the card-like paper was attached, and towards the opposite side, bearing the very thin paper. This occurred in some instances in 9 h., but in others not until 24 h. had elapsed. Moreover, some of the four failures can hardly be considered as really failures: thus, in one of them, in which the radicle remained quite straight, the square of thin paper was found, when both were removed from the apex, to have been so thickly coated with shellac that it was almost as stiff as the card: in the second case, the radicle was bent upwards into a semicircle, but the deflection was not directly from the side bearing the card, and this was explained by the two squares having become cemented laterally together, forming a sort of stiff gable, from which the radicle was deflected: in the third case, the square of card had been fixed by mistake in front, and though there was deflection from it, this might have been due to Sachs' curvature: [page 149] in the fourth case alone no reason could be assigned why the radicle had not been at all deflected. These experiments suffice to prove that the apex of the radicle possesses the extraordinary power of discriminating between thin card and very thin paper, and is deflected from the side pressed by the more resisting or harder substance.

Some trials were next made by irritating the tips without any object being left in contact with them. Nine radicles, suspended over water, had their tips rubbed, each six times with a needle, with sufficient force to shake the whole bean; the temperature was favourable, viz. about 63o F. In 7 out of these cases no effect whatever was produced; in the eighth case the radicle became slightly deflected from, and in the ninth case slightly deflected towards, the rubbed side; but these two latter opposed curvatures were probably accidental, as radicles do not always grow perfectly straight downwards. The tips of two other radicles were rubbed in the same manner for 15 seconds with a little round twig, two others for 30 seconds, and two others for 1 minute, but without any effect being produced. We may therefore conclude from these 15 trials that the radicles are not sensitive to temporary contact, but are acted on only by prolonged, though very slight, pressure.

We then tried the effects of cutting off a very thin slice parallel to one of the sloping sides of the apex, as we thought that the wound would cause prolonged irritation, which might induce bending towards the opposite side, as in the case of an attached object. Two preliminary trials were made: firstly, slices were cut from the radicles of 6 beans suspended in damp air, with a pair of scissors, which, though sharp, probably caused considerable crushing, and no curva- [page 150] ture followed. Secondly, thin slices were cut with a razor obliquely off the tips of three radicles similarly suspended; and after 44 h. two were found plainly bent from the sliced surface; and the third, the whole apex of which had been cut off obliquely by accident, was curled upwards over the bean, but it was not clearly ascertained whether the curvature had been at first directed from the cut surface. These results led us to pursue the experiment, and 18 radicles, which had grown vertically downwards in damp air, had one side of their conical tips sliced off with a razor. The tips were allowed just to enter the water in the jars, and they were exposed to a temperature 14o - 16o C. (57o - 61o F.). The observations were made at different times. Three were examined 12 h. after being sliced, and were all slightly curved from the cut surface; and the curvature increased considerably after an additional 12 h. Eight were examined after 19 h.; four after 22 h. 30 m.; and three after 25 h. The final result was that out of the 18 radicles thus tried, 13 were plainly bent from the cut surface after the above intervals of time; and one other became so after an additional interval of 13 h. 30 m. So that only 4 out of the 18 radicles were not acted on. To these 18 cases the 3 previously mentioned ones should be added. It may, therefore, be concluded that a thin slice removed by a razor from one side of the conical apex of the radicle causes irritation, like that from an attached object, and induces curvature from the injured surface.

Lastly, dry caustic (nitrate of silver) was employed to irritate one side of the apex. If one side of the apex or of the whole terminal growing part of a radicle, is by any means killed or badly injured, the other side continues to grow; and this causes the part [page 151] to bend over towards the injured side.* But in the following experiments we endeavoured, generally with success, to irritate the tips on one side, without badly injuring them. This was effected by first drying the tip as far as possible with blotting-paper, though it still remained somewhat damp, and then touching it once with quite dry caustic. Seventeen radicles were thus treated, and were suspended in moist air over water at a temperature of 58o F. They were examined after an interval of 21 h. or 24h. The tips of two were found blackened equally all round, so that they could tell nothing and were rejected, 15 being left. Of these, 10 were curved from the side which had been touched, where there was a minute brown or blackish mark. Five of these radicles, three of which were already slightly deflected, were allowed to enter the water in the jar, and were re-examined after an additional interval of 27 h. (i.e. in 48 h. after the application of the caustic), and now four of them had become hooked, being bent from the discoloured side, with their points directed to the zenith; the fifth remained unaffected and straight. Thus 11 radicles out of the 15 were acted on. But the curvature of the four just described was so plain, that they alone would have sufficed to show that the radicles of the bean bend away from that side of the apex which has been slightly irritated by caustic.

The Power of an Irritant on the apex of the Radicle

* Ciesielski found this to be the case ('Untersuchungen ber die Abwartskrmmung der Wurzel,' 1871, p. 28) after burning with heated platinum one side of a radicle. So did we when we painted longitudinally half of the whole length of 7 radicles, suspended over water, with a thick layer of grease, which is very injurious or even fatal to growing parts; for after 48 hours five of these radicles were curved towards the greased side, two remaining straight. [page 152]

of the Bean, compared with that of Geotropism.—We know that when a little square of card or other object is fixed to one side of the tip of a vertically dependent radicle, the growing part bends from it often into a semicircle, in opposition to geotropism, which force is conquered by the effect of the irritation from the attached object. Radicles were therefore extended horizontally in damp air, kept at the proper low temperature for full sensitiveness, and squares of card were affixed with shellac on the lower sides of their tips, so that if the squares acted, the terminal growing part would curve upwards. Firstly, eight beans were so placed that their short, young, horizontally extended radicles would be simultaneously acted on both by geotropism and by Sachs' curvature, if the latter came into play; and they all eight became bowed downwards to the centre of the earth in 20 h., excepting one which was only slightly acted on. Two of them were a little bowed downwards in only 5 h.! Therefore the cards, affixed to the lower sides of their tips, seemed to produce no effect; and geotropism easily conquered the effects of the irritation thus caused. Secondly, 5 oldish radicles, 1 inch in length, and therefore less sensitive than the above-mentioned young ones, were similarly placed and similarly treated. From what has been seen on many other occasions, it may be safely inferred that if they had been suspended vertically they would have bent away from the cards; and if they had been extended horizontally, without cards attached to them, they would have quickly bent vertically downwards through geotropism; but the result was that two of these radicles were still horizontal after 23 h.; two were curved only slightly, and the fifth as much as 40o beneath the horizon. Thirdly, 5 beans were fastened [page 153] with their flat surfaces parallel to the cork-lid, so that Sachs' curvature would not tend to make the horizontally extended radicles turn either upwards or downwards, and little squares of card were affixed as before, to the lower sides of their tips. The result was that all five radicles were bent down, or towards the centre of the earth, after only 8 h. 20 m. At the same time and within the same jars, 3 radicles of the same age, with squares affixed to one side, were suspended vertically; and after 8 h. 20 m. they were considerably deflected from the cards, and therefore curved upwards in opposition to geotropism. In these latter cases the irritation from the squares had over-powered geotropism; whilst in the former cases, in which the radicles were extended horizontally, geotropism had overpowered the irritation. Thus within the same jars, some of the radicles were curving upwards and others downwards at the same time—these opposite movements depending on whether the radicles, when the squares were first attached to them, projected vertically down, or were extended horizontally. This difference in their behaviour seems at first inexplicable, but can, we believe, be simply explained by the difference between the initial power of the two forces under the above circumstances, combined with the well-known principle of the after-effects of a stimulus. When a young and sensitive radicle is extended horizontally, with a square attached to the lower side of the tip, geotropism acts on it at right angles, and, as we have seen, is then evidently more efficient than the irritation from the square; and the power of geotropism will be strengthened at each successive period by its previous action—that is, by its after-effects. On the other hand, when a square is affixed to a vertically dependent radicle, and the apex begins to [page 154] curve upwards, this movement will be opposed by geotropism acting only at a very oblique angle, and the irritation from the card will be strengthened by its previous action. We may therefore conclude that the initial power of an irritant on the apex of the radicle of the bean, is less than that of geotropism when acting at right angles, but greater than that of geotropism when acting obliquely on it.

Sensitiveness of the tips of the Secondary Radicles of the Bean to contact.—All the previous observations relate to the main or primary radicle. Some beans suspended to cork-lids, with their radicles dipping into water, had developed secondary or lateral radicles, which were afterwards kept in very damp air, at the proper low temperature for full sensitiveness. They projected, as usual, almost horizontally, with only a slight downward curvature, and retained this position during several days. Sachs has shown* that these secondary roots are acted on in a peculiar manner by geotropism, so that if displaced they reassume their former sub-horizontal position, and do not bend vertically downwards like the primary radicle. Minute squares of the stiff sanded paper were affixed by means of shellac (but in some instances with thick gum-water) to the tips of 39 secondary radicles of different ages, generally the uppermost ones. Most of the squares were fixed to the lower sides of the apex, so that if they acted the radicle would bend upwards; but some were fixed laterally, and a few on the upper side. Owing to the extreme tenuity of these radicles, it was very difficult to attach the square to the actual apex. Whether owing to this or some other circumstance, only nine of the squares induced any * 'Arbeiten Bot. Inst., Wrzburg,' Heft iv. 1874, p. 605-617. [page 155]

curvature. The curvature amounted in some cases to about 45o above the horizon, in others to 90o, and then the tip pointed to the zenith. In one instance a distinct upward curvature was observed in 8 h. 15 m., but usually not until 24 h. had elapsed. Although only 9 out of 39 radicles were affected, yet the curvature was so distinct in several of them, that there could be no doubt that the tip is sensitive to slight contact, and that the growing part bends away from the touching object. It is possible that some secondary radicles are more sensitive than others; for Sachs has proved* the interesting fact that each individual secondary radicle possesses its own peculiar constitution.

Sensitiveness to contact of the Primary Radicle, a little above the apex, in the Bean (Vicia faba) and Pea (Pisum sativum).—The sensitiveness of the apex of the radicle in the previously described cases, and the consequent curvature of the upper part from the touching object or other source of irritation, is the more remarkable, because Sachs** has shown that pressure at the distance of a few millimeters above the apex causes the radicle to bend, like a tendril, towards the touching object. By fixing pins so that they pressed against the radicles of beans suspended vertically in damp air, we saw this kind of curvature; but rubbing the part with a twig or needle for a few minutes produced no effect. Haberlandt remarks,*** that these radicles in breaking through the seed-coats often rub and press against the ruptured edges, and consequently bend round them. As little squares of the card-like paper affixed with shellac to the tips were highly efficient in causing the radicles to bend away from them, similar pieces (of about 1/20th

* 'Arbeiten Bot. Instit., Wrzburg,' Heft, iv. 1874, p. 620.

** Ibid. Heft iii. 1873, p. 437.

*** 'Die Schutzeinrichtungen der Keimpflanze,' 1877, p. 25. [page 156]

inch square, or rather less) were attached in the same manner to one side of the radicle at a distance of 3 or 4 mm. above the apex. In our first trial on 15 radicles no effect was produced. In a second trial on the same number, three became abruptly curved (but only one strongly) towards the card within 24 h. From these cases we may infer that the pressure from a bit of card affixed with shellac to one side above the apex, is hardly a sufficient irritant; but that it occasionally causes the radicle to bend like a tendril towards this side.

We next tried the effect of rubbing several radicles at a distance of 4 mm. from the apex for a few seconds with lunar caustic (nitrate of silver); and although the radicles had been wiped dry and the stick of caustic was dry, yet the part rubbed was much injured and a slight permanent depression was left. In such cases the opposite side continues to grow, and the radicle necessarily becomes bent towards the injured side. But when a point 4 mm. from the apex was momentarily touched with dry caustic, it was only faintly discoloured, and no permanent injury was caused. This was shown by several radicles thus treated straightening themselves after one or two days; yet at first they became curved towards the touched side, as if they had been there subjected to slight continued pressure. These cases deserve notice, because when one side of the apex was just touched with caustic, the radicle, as we have seen, curved itself in an opposite direction, that is, away from the touched side.

The radicle of the common pea at a point a little above the apex is rather more sensitive to continued pressure than that of the bean, and bends towards the pressed side.* We experimented on a variety (York-

* Sachs, 'Arbeiten Bot. Institut., Wrzburg,' Heft iii. p. 438. [page 157]

shire Hero) which has a much wrinkled tough skin, too large for the included cotyledons; so that out of 30 peas which had been soaked for 24 h. and allowed to germinate on damp sand, the radicles of three were unable to escape, and were crumpled up in a strange manner within the skin; four other radicles were abruptly bent round the edges of the ruptured skin against which they had pressed. Such abnormalities would probably never, or very rarely, occur with forms developed in a state of nature and subjected to natural selection. One of the four radicles just mentioned in doubling backwards came into contact with the pin by which the pea was fixed to the cork-lid; and now it bent at right angles round the pin, in a direction quite different from that of the first curvature due to contact with the ruptured skin; and it thus afforded a good illustration of the tendril-like sensitiveness of the radicle a little above the apex.

Little squares of the card-like paper were next affixed to radicles of the pea at 4 mm. above the apex, in the same manner as with the bean. Twenty-eight radicles suspended vertically over water were thus treated on different occasions, and 13 of them became curved towards the cards. The greatest degree of curvature amounted to 62o from the perpendicular; but so large an angle was only once formed. On one occasion a slight curvature was perceptible after 5 h. 45 m., and it was generally well-marked after 14 h. There can therefore be no doubt that with the pea, irritation from a bit of card attached to one side of the radicle above the apex suffices to induce curvature.

Squares of card were attached to one side of the tips of 11 radicles within the same jars in which the above trials were made, and five of them became plainly, and one slightly, curved away from this side. Other [page 158] analogous cases will be immediately described. The fact is here mentioned because it was a striking spectacle, showing the difference in the sensitiveness of the radicle in different parts, to behold in the same jar one set of radicles curved away from the squares on their tips, and another set curved towards the squares attached a little higher up. Moreover, the kind of curvature in the two cases is different. The squares attached above the apex cause the radicle to bend abruptly, the part above and beneath remaining nearly straight; so that here there is little or no transmitted effect. On the other hand, the squares attached to the apex affect the radicle for a length of from about 4 to even 8 mm., inducing in most cases a symmetrical curvature; so that here some influence is transmitted from the apex for this distance along the radicle.

Pisum sativum (var. Yorkshire Hero): Sensitiveness of the apex of the Radicle.—Little squares of the same card-like paper were affixed (April 24th) with shellac to one side of the apex of 10 vertically suspended radicles: the temperature of the water in the bottom of the jars was 60o - 61o F. Most of these radicles were acted on in 8 h. 30 m.; and eight of them became in the course of 24 h. conspicuously, and the remaining two slightly, deflected from the perpendicular and from the side bearing the attached squares. Thus all were acted on; but it will suffice to describe two conspicuous cases. In one the terminal portion of the radicle was bent at right angles (A, Fig. 66) after 24h.; and in the other (B) it had by this time become hooked, with the apex pointing to the zenith. The two bits of card here used were .07 inch in length and .04 inch in breadth. Two other radicles, which after 8 h. 30 m. were moderately deflected, became straight again after 24h. Another [page 159] trial was made in the same manner with 15 radicles; but from circumstances, not worth explaining, they were only once and briefly examined after the short

Fig. 66. Pisum sativum: deflection produced within 24 hours in the growth of vertically dependent radicles, by little squares of card affixed with shellac to one side of apex: A, bent at right angles; B, hooked.

interval of 5 h. 30 m.; and we merely record in our notes "almost all bent slightly from the perpendicular, and away from the squares; the deflection amounting in one or two instances to nearly a rectangle." These two sets of cases, especially the first one, prove that the apex of the radicle is sensitive to slight contact and that the upper part bends from the touching object. Nevertheless, on June 1st and 4th, 8 other radicles were tried in the same manner at a temperature of 58o - 60o F., and after 24 h. only 1 was decidedly bent from the card, 4 slightly, 2 doubtfully, and 1 not in the least. The amount of curvature was unaccountably small; but all the radicles which were at all bent, were bent away from the cards.

We now tried the effects of widely different temperatures on the sensitiveness of these radicles with squares [page 160] of card attached to their tips. Firstly, 13 peas, most of them having very short and young radicles, were placed in an ice-box, in which the temperature rose during three days from 44o to 47o F. They grew slowly, but 10 out of the 13 became in the course of the three days very slightly curved from the squares; the other 3 were not affected; so that this temperature was too low for any high degree of sensitiveness or for much movement. Jars with 13 other radicles were next placed on a chimney-piece, where they were subjected to a temperature of between 68o and 72o F., and after 24 h., 4 were conspicuously curved from the cards, 2 slightly, and 7 not at all; so that this temperature was rather too high. Lastly 12 radicles were subjected to a temperature varying between 72o and 85o F., and none of them were in the least affected by the squares. The above several trials, especially the first recorded one, indicate that the most favourable temperature for the sensitiveness of the radicle of the pea is about 60o F.

The tips of 6 vertically dependent radicles were touched once with dry caustic, in the manner described under Vicia faba. After 24 h. four of them were bent from the side bearing a minute black mark; and the curvature increased in one case after 38 h., and in another case after 48 h., until the terminal part projected almost horizontally. The two remaining radicles were not affected.

With radicles of the bean, when extended horizontally in damp air, geotropism always conquered the effects of the irritation caused by squares of card attached to the lower sides of their tips. A similar experiment was tried on 13 radicles of the pea; the squares being attached with shellac, and the temperature between 58o - 60o F. The result was somewhat different; for [page 161] these radicles are either less strongly acted on by geotropism, or, what is more probable, are more sensitive to contact. After a time geotropism always prevailed, but its action was often delayed; and in three instances there was a most curious struggle between geotropism and the irritation caused by the cards. Four of the 13 radicles were a little curved downwards within 6 or 8 h., always reckoning from the time when the squares were first attached, and after 23 h. three of them pointed vertically downwards, and the fourth at an angle of 45o beneath the horizon. These four radicles therefore did not seem

Fig. 67. Pisum sativum: a radicle extended horizontally in damp air with a little square of card affixed to the lower side of its tip, causing it to bend upwards in opposition to geotropism. The deflection of the radicle after 21 hours is shown at A, and of the same radicle after 45 hours at B, now forming a loop.

to have been at all affected by the attached squares. Four others were not acted on by geotropism within the first 6 or 8 h., but after 23 h. were much bowed down. Two others remained almost horizontal for 23 h., but afterwards were acted on. So that in these latter six cases the action of geotropism was much delayed. The eleventh radicle was slightly curved down after 8 h., but when looked at again after 23 h. the terminal portion was curved upwards; if it had [page 162] been longer observed, the tip no doubt would have been found again curved down, and it would have formed a loop as in the following case. The twelfth radicle after 6 h. was slightly curved downwards; but when looked at again after 21 h., this curvature had disappeared and the apex pointed upwards; after 30 h. the radicle formed a hook, as shown at A (Fig. 67); which hook after 45 h. was converted into a loop (B). The thirteenth radicle after 6 h. was slightly curved downwards, but within 21 h. had curved considerably up, and then down again at an angle of 45o beneath the horizon, afterwards becoming perpendicular. In these three last cases geotropism and the irritation caused by the attached squares alternately prevailed in a highly remarkable manner; geotropism being ultimately victorious.

Similar experiments were not always quite so successful as in the above cases. Thus 6 radicles, horizontally extended with attached squares, were tried on June 8th at a proper temperature, and after 7 h. 30 m. none were in the least curved upwards and none were distinctly geotropic; whereas of 6 radicles without any attached squares, which served as standards of comparison or controls, 3 became slightly and 3 almost rectangularly geotropic within the 7 h. 30 m.; but after 23 h. the two lots were equally geotropic. On July 10th another trial was made with 6 horizontally extended radicles, with squares attached in the same manner beneath their tips; and after 7 h. 30 m., 4 were slightly geotropic, 1 remained horizontal, and 1 was curved upwards in opposition to gravity or geotropism. This latter radicle after 48 h. formed a loop, like that at B (Fig. 67).

An analogous trial was now made, but instead of attaching squares of card to the lower sides of the [page 163] tips, these were touched with dry caustic. The details of the experiment will be given in the chapter on Geotropism, and it will suffice here to say that 10 peas, with radicles extended horizontally and not cauterised, were laid on and under damp friable peat; these, which served as standards or controls, as well as 10 others which had been touched on the upper side with the caustic, all became strongly geotropic in 24 h. Nine radicles, similarly placed, had their tips touched on the lower side with the caustic; and after 24 h., 3 were slightly geotropic, 2 remained horizontal, and 4 were bowed upwards in opposition to gravity and to geotropism. This upward curvature was distinctly visible in 8 h. 45m. after the lower sides of the tips had been cauterised.

Little squares of card were affixed with shellac on two occasions to the tips of 22 young and short secondary radicles, which had been emitted from the primary radicle whilst growing in water, but were now suspended in damp air. Besides the difficulty of attaching the squares to such finely pointed objects as were these radicles, the temperature was too high,—varying on the first occasion from 72o to 77o F., and on the second being almost steadily 78o F.; and this probably lessened the sensitiveness of the tips. The result was that after an interval of 8 h. 30 m., 6 of the 22 radicles were bowed upwards (one of them greatly) in opposition to gravity, and 2 laterally; the remaining 14 were not affected. Considering the unfavourable circumstances, and bearing in mind the case of the bean, the evidence appears sufficient to show that the tips of the secondary radicles of the pea are sensitive to slight contact.

Phaseolus multiflorus: Sensitiveness of the apex of the Radicle.— Fifty-nine radicles were tried with squares [page 164] of various sizes of the same card-like paper, also with bits of thin glass and rough cinders, affixed with shellac to one side of the apex. Rather large drops of the dissolved shellac were also placed on them and allowed to set into hard beads. The specimens were subjected to various temperatures between 60o and 72o F., more commonly at about the latter. But out of this considerable number of trials only 5 radicles were plainly bent, and 8 others slightly or even doubtfully, from the attached objects; the remaining 46 not being at all affected. It is therefore clear that the tips of the radicles of this Phaseolus are much less sensitive to contact than are those of the bean or pea. We thought that they might be sensitive to harder pressure, but after several trials we could not devise any method for pressing harder on one side of the apex than on the other, without at the same time offering mechanical resistance to its growth. We therefore tried other irritants.

The tips of 13 radicles, dried with blotting-paper, were thrice touched or just rubbed on one side with dry nitrate of silver. They were rubbed thrice, because we supposed from the foregoing trials, that the tips were not highly sensitive. After 24 h. the tips were found greatly blackened; 6 were blackened equally all round, so that no curvature to any one side could be expected; 6 were much blackened on one side for a length of about 1/10th of an inch, and this length became curved at right angles towards the blackened surface, the curvature afterwards increasing in several instances until little hooks were formed. It was manifest that the blackened side was so much injured that it could not grow, whilst the opposite side continued to grow. One alone out of these 13 radicles became curved from the blackened side, the [page 165] curvature extending for some little distance above the apex.

After the experience thus gained, the tips of six almost dry radicles were once touched with the dry caustic on one side; and after an interval of 10 m. were allowed to enter water, which was kept at a temperature of 65o - 67o F. The result was that after an interval of 8 h. a minute blackish speck could just be distinguished on one side of the apex of five of these radicles, all of which became curved towards the opposite side—in two cases at about an angle of 45o—in two other cases at nearly a rectangle— and in the fifth case at above a rectangle, so that the apex was a little hooked; in this latter case the black mark was rather larger than in the others. After 24 h. from the application of the caustic, the curvature of three of these radicles (including the hooked one) had diminished; in the fourth it remained the same, and in the fifth it had increased, the tip being now hooked. It has been said that after 8 h. black specks could be seen on one side of the apex of five of the six radicles; on the sixth the speck, which was extremely minute, was on the actual apex and therefore central; and this radicle alone did not become curved. It was therefore again touched on one side with caustic, and after 15 h. 30 m. was found curved from the perpendicular and from the blackened side at an angle of 34o, which increased in nine additional hours to 54o.

It is therefore certain that the apex of the radicle of this Phaseolus is extremely sensitive to caustic, more so than that of the bean, though the latter is far more sensitive to pressure. In the experiments just given, the curvature from the slightly cauterised side of the tip, extended along the radicle for a length of nearly 10 mm.; whereas in the first set [page 166] of experiments, when the tips of several were greatly blackened and injured on one side, so that their growth was arrested, a length of less than 3 mm. became curved towards the much blackened side, owing to the continued growth of the opposite side. This difference in the results is interesting, for it shows that too strong an irritant does not induce any transmitted effect, and does not cause the adjoining, upper and growing part of the radicle to bend. We have analogous cases with Drosera, for a strong solution of carbonate of ammonia when absorbed by the glands, or too great heat suddenly applied to them, or crushing them, does not cause the basal part of the tentacles to bend, whilst a weak solution of the carbonate, or a moderate heat, or slight pressure always induced such bending. Similar results were observed with Dionaea and Pinguicula.

The effect of cutting off with a razor a thin slice from one side of the conical apex of 14 young and short radicles was next tried. Six of them after being operated on were suspended in damp air; the tips of the other eight, similarly suspended, were allowed to enter water at a temperature of about 65o F. It was recorded in each case which side of the apex had been sliced off, and when they were afterwards examined the direction of the curvature was noted, before the record was consulted. Of the six radicles in damp air, three had their tips curved after an interval of 10 h. 15 m. directly away from the sliced surface, whilst the other three were not affected and remained straight; nevertheless, one of them after 13 additional hours became slightly curved from the sliced surface. Of the eight radicles with their tips immersed in water, seven were plainly curved away from the sliced surfaces after 10 h. 15 m.; and with [page 167] respect to the eighth which remained quite straight, too thick a slice had been accidentally removed, so that it hardly formed a real exception to the general result. When the seven radicles were looked at again, after an interval of 23 h. from the time of slicing, two had become distorted; four were deflected at an angle of about 70o from the perpendicular and from the cut surface; and one was deflected at nearly 90o, so that it projected almost horizontally, but with the extreme tip now beginning to bend downwards through the action of geotropism. It is therefore manifest that a thin slice cut off one side of the conical apex, causes the upper growing part of the radicle of this Phaseolus to bend, through the transmitted effects of the irritation, away from the sliced surface.

Tropaeolum majus: Sensitiveness of the apex of the Radicle to contact.— Little squares of card were attached with shellac to one side of the tips of 19 radicles, some of which were subjected to 78o F., and others to a much lower temperature. Only 3 became plainly curved from the squares, 5 slightly, 4 doubtfully, and 7 not at all. These seeds were, as we believed, old, so we procured a fresh lot, and now the results were widely different. Twenty-three were tried in the same manner; five of the squares produced no effect, but three of these cases were no real exceptions, for in two of them the squares had slipped and were parallel to the apex, and in the third the shellac was in excess and had spread equally all round the apex. One radicle was deflected only slightly from the perpendicular and from the card; whilst seventeen were plainly deflected. The angles in several of these latter cases varied between 40o and 65o from the perpendicular; and in two of them it amounted after 15 h. or 16 h. to about 90o. In one instance a loop [page 168] was nearly completed in 16 h. There can, therefore, be no doubt that the apex is highly sensitive to slight contact, and that the upper part of the radicle bends away from the touching object.

Gossypium herbaceum: Sensitiveness of the apex of the Radicle.—Radicles were experimented on in the same manner as before, but they proved ill-fitted for our purpose, as they soon became unhealthy when suspended in damp air. Of 38 radicles thus suspended, at temperatures varying from 66o to 69o F., with squares of card attached to their tips, 9 were plainly and 7 slightly or even doubtfully deflected from the squares and from the perpendicular; 22 not being affected. We thought that perhaps the above temperature was not high enough, so 19 radicles with attached squares, likewise suspended in damp air, were subjected to a temperature of from 74o to 79o F., but not one of them was acted on, and they soon became unhealthy. Lastly, 19 radicles were suspended in water at a temperature from 70o to 75o F., with bits of glass or squares of the card attached to their tips by means of Canada-balsam or asphalte, which adhered rather better than shellac beneath the water. The radicles did not keep healthy for long. The result was that 6 were plainly and 2 doubtfully deflected from the attached objects and the perpendicular; 11 not being affected. The evidence consequently is hardly conclusive, though from the two sets of cases tried under a moderate temperature, it is probable that the radicles are sensitive to contact; and would be more so under favourable conditions.

Fifteen radicles which had germinated in friable peat were suspended vertically over water. Seven of them served as controls, and they remained quite straight during 24 h. The tips of the other eight radicles [page 169] were just touched with dry caustic on one side. After only 5 h. 10 m. five of them were slightly curved from the perpendicular and from the side bearing the little blackish marks. After 8 h. 40 m., 4 out of these 5 were deflected at angles between 15o and 65o from the perpendicular. On the other hand, one which had been slightly curved after 5 h. 10 m., now became straight. After 24 h. the curvature in two cases had considerably increased; also in four other cases, but these latter radicles had now become so contorted, some being turned upwards, that it could no longer be ascertained whether they were still curved from the cauterised side. The control specimens exhibited no such irregular growth, and the two sets presented a striking contrast. Out of the 8 radicles which had been touched with caustic, two alone were not affected, and the marks left on their tips by the caustic were extremely minute. These marks in all cases were oval or elongated; they were measured in three instances, and found to be of nearly the same size, viz. 2/3 of a mm. in length. Bearing this fact in mind, it should be observed that the length of the curved part of the radicle, which had become deflected from the cauterised side in the course of 8 h. 40 m. was found to be in three cases 6, 7, and 9 mm.

Cucurbita ovifera: Sensitiveness of the apex of the Radicle.—The tips proved ill-fitted for the attachment of cards, as they are extremely fine and flexible. Moreover, owing to the hypocotyls being soon developed and becoming arched, the whole radicle is quickly displaced and confusion is thus caused. A large number of trials were made, but without any definite result, excepting on two occasions, when out of 23 radicles 10 were deflected from the attached squares [page 170] of card, and 13 were not acted on. Rather large squares, though difficult to affix, seemed more efficient than very small ones.

We were much more successful with caustic; but in our first trial, 15 radicles were too much cauterised, and only two became curved from the blackened side; the others being either killed on one side, or blackened equally all round. In our next trial the dried tips of 11 radicles were touched momentarily with dry caustic, and after a few minutes were immersed in water. The elongated marks thus caused were never black, only brown, and about mm. in length, or even less. In 4 h. 30 m. after the cauterisation, 6 of them were plainly curved from the side with the brown mark, 4 slightly, and 1 not at all. The latter proved unhealthy, and never grew; and the marks on 2 of the 4 slightly curved radicles were excessively minute, one being distinguishable only with the aid of a lens. Of 10 control specimens tried in the same jars at the same time, not one was in the least curved. In 8 h. 40 m. after the cauterisation, 5 of the radicles out of the 10 (the one unhealthy one being omitted) were deflected at about 90o, and 3 at about 45o from the perpendicular and from the side bearing the brown mark. After 24 h. all 10 radicles had increased immensely in length; in 5 of them the curvature was nearly the same, in 2 it had increased, and in 3 it had decreased. The contrast presented by the 10 controls, after both the 8 h. 40 m. and the 24 h. intervals, was very great; for they had continued to grow vertically downwards, excepting two which, from some unknown cause, had become somewhat tortuous.

In the chapter on Geotropism we shall see that 10 radicles of this plant were extended horizontally on and beneath damp friable peat, under which conditions [page 171] they grow better and more naturally than in damp air; and their tips were slightly cauterised on the lower side, brown marks about mm. in length being thus caused. Uncauterised specimens similarly placed became much bent downwards through geotropism in the course of 5 or 6 hours. After 8 h. only 3 of the cauterised ones were bowed downwards, and this in a slight degree; 4 remained horizontal; and 3 were curved upwards in opposition to geotropism and from the side bearing the brown mark. Ten other specimens had their tips cauterised at the same time and in the same degree, on the upper side; and this, if it produced any effect, would tend to increase the power of geotropism; and all these radicles were strongly bowed downwards after 8 h. From the several foregoing facts, there can be no doubt that the cauterisation of the tip of the radicle of this Cucurbita on one side, if done lightly enough, causes the whole growing part to bend to the opposite side. Raphanus sativus: Sensitiveness of the apex of the Radicle.—We here encountered many difficulties in our trials, both with squares of card and with caustic; for when seeds were pinned to a cork-lid, many of the radicles, to which nothing had been done, grew irregularly, often curving upwards, as if attracted by the damp surface above; and when they were immersed in water they likewise often grew irregularly. We did not therefore dare to trust our experiments with attached squares of card; nevertheless some of them seemed to indicate that the tips were sensitive to contact. Our trials with caustic generally failed from the difficulty of not injuring too greatly the extremely fine tips. Out of 7 radicles thus tried, one became bowed after 22 h. at an angle of 60o, a second at 40o, [page 172] and a third very slightly from the perpendicular and from the cauterised side.

Aesculus hippocastanum: Sensitiveness of the apex of the Radicle.—Bits of glass and squares of card were affixed with shellac or gum-water to the tips of 12 radicles of the horse-chestnut; and when these objects fell off, they were refixed; but not in a single instance was any curvature thus caused. These massive radicles, one of which was above 2 inches in length and .3 inch in diameter at its base, seemed insensible to so slight a stimulus as any small attached object. Nevertheless, when the apex encountered an obstacle in its downward course, the growing part became so uniformly and symmetrically curved, that its appearance indicated not mere mechanical bending, but increased growth along the whole convex side, due to the irritation of the apex.

That this is the correct view may be inferred from the effects of the more powerful stimulus of caustic. The bending from the cauterised side occurred much slower than in the previously described species, and it will perhaps be worth while to give our trials in detail.

[The seeds germinated in sawdust, and one side of the tips of the radicles were slightly rubbed once with dry nitrate of silver; and after a few minutes were allowed to dip into water. They were subjected to a rather varying temperature, generally between 52o and 58o F. A few cases have not been thought worth recording, in which the whole tip was blackened, or in which the seedling soon became unhealthy.

(1.) The radicle was slightly deflected from the cauterised side in one day (i.e. 24 h.); in three days it stood at 60o from the perpendicular; in four days at 90o; on the fifth day it was curved up about 40o above the horizon; so that it had passed through an angle of 130o in the five days, and this was the greatest amount of curvature observed.

(2.) In two days radicle slightly deflected; after seven days [page 173] deflected 69o from the perpendicular and from the cauterised side; after eight days the angle amounted to nearly 90o.

(3.) After one day slight deflection, but the cauterised mark was so faint that the same side was again touched with caustic. In four days from the first touch deflection amounted to 78o, which in an additional day increased to 90o.

(4.) After two days slight deflection, which during the next three days certainly increased but never became great; the radicle did not grow well and died on the eighth day.

(5.) After two days very slight deflection; but this on the fourth day amounted to 56o from the perpendicular and from the cauterised side.

(6.) After three days doubtfully, but after four days certainly deflected from the cauterised side. On the fifth day deflection amounted to 45o from the perpendicular, and this on the seventh day increased to about 90o.

(7.) After two days slightly deflected; on the third day the deflection amounted to 25o from the perpendicular, and this did not afterwards increase.

(8.) After one day deflection distinct; on the third day it amounted to 44o, and on the fourth day to 72o from the perpendicular and the cauterised side.

(9.) After two days deflection slight, yet distinct; on the third day the tip was again touched on the same side with caustic and thus killed.

(10.) After one day slight deflection, which after six days increased to 50o from the perpendicular and the cauterised side.

(11.) After one day decided deflection, which after six days increased to 62o from the perpendicular and from the cauterised side.

(12.) After one day slight deflection, which on the second day amounted to 35o, on the fourth day to 50o, and the sixth day to 63o from the perpendicular and the cauterised side.

(13.) Whole tip blackened, but more on one side than the other; on the fourth day slightly, and on the sixth day greatly deflected from the more blackened side; the deflection on the ninth day amounted to 90o from the perpendicular.

(14.) Whole tip blackened in the same manner as in the last case: on the second day decided deflection from the more blackened side, which increased on the seventh day to nearly 90o; on the following day the radicle appeared unhealthy.

(15.) Here we had the anomalous case of a radicle bending [page 174] slightly towards the cauterised side on the first day, and continuing to do so for the next three days, when the deflection amounted to about 90o from the perpendicular. The cause appeared to lie in the tendril-like sensitiveness of the upper part of the radicle, against which the point of a large triangular flap of the seed-coats pressed with considerable force; and this irritation apparently conquered that from the cauterised apex.]

These several cases show beyond doubt that the irritation of one side of the apex, excites the upper part of the radicle to bend slowly towards the opposite side. This fact was well exhibited in one lot of five seeds pinned to the cork-lid of a jar; for when after 6 days the lid was turned upside down and viewed from directly above, the little black marks made by the caustic were now all distinctly visible on the upper sides of the tips of the laterally bowed radicles. A thin slice was shaved off with a razor from one side of the tips of 22 radicles, in the manner described under the common bean; but this kind of irritation did not prove very effective. Only 7 out of the 22 radicles became moderately deflected in from 3 to 5 days from the sliced surface, and several of the others grew irregularly. The evidence, therefore, is far from conclusive.

Quercus robur: Sensitiveness of the apex of the Radicle.—The tips of the radicles of the common oak are fully as sensitive to slight contact as are those of any plant examined by us. They remained healthy in damp air for 10 days, but grew slowly. Squares of the card-like paper were fixed with shellac to the tips of 15 radicles, and ten of these became conspicuously bowed from the perpendicular and from the squares; two slightly, and three not at all. But two of the latter were not real exceptions, as they were at first very short, and hardly grew afterwards. Some of the more [page 175] remarkable cases are worth describing. The radicles were examined on each successive morning, at nearly the same hour, that is, after intervals of 24 h.

[No. 1. This radicle suffered from a series of accidents, and acted in an anomalous manner, for the apex appeared at first insensible and afterwards sensitive to contact. The first square was attached on Oct 19th; on the 21st the radicle was not at all curved, and the square was accidentally knocked off; it was refixed on the 22nd, and the radicle became slightly curved from the square, but the curvature disappeared on the 23rd, when the square was removed and refixed. No curvature ensued, and the square was again accidentally knocked off, and refixed. On the morning of the 27th it was washed off by having reached the water in the bottom of the jar. The square was refixed, and on the 29th, that is, ten days after the first square had been attached, and two days after the attachment of the last square, the radicle had grown to the great length of 3.2 inches, and now the terminal growing part had become bent away from the square into a hook (see Fig. 68).

Fig. 68. Quercus robur: radicle with square of card attached to one side of apex, causing it to become hooked. Drawing one-half natural scale.

No. 2. Square attached on the 19th; on the 20th radicle slightly deflected from it and from the perpendicular; on the 21st deflected at nearly right angles; it remained during the next two days in this position, but on the 25th the upward curvature was lessened through the action of geotropism, and still more so on the 26th.

No. 3. Square attached on the 19th; on the 21st a trace of curvature from the square, which amounted on the 22nd to about 40o, and on the 23rd to 53o from the perpendicular.

No. 4. Square attached on the 21st; on the 22nd trace of curvature from the square; on the 23rd completely hooked with the point turned up to the zenith. Three days afterwards (i.e. 26th) the curvature had wholly disappeared and the apex pointed perpendicularly downwards.

No. 5. Square attached on the 21st; on the 22nd decided [page 176] though slight curvature from the square; on the 23rd the tip had curved up above the horizon, and on the 24th was hooked with the apex pointing almost to the zenith, as in Fig. 68.

No. 6. Square attached on the 21st; on the 22nd slightly curved from the square; 23rd more curved; 25th considerably curved; 27th all curvature lost, and the radicle was now directed perpendicularly downwards.

No. 7. Square attached on the 21st; on the 22nd a trace of curvature from the square, which increased next day, and on the 24th amounted to a right angle.

It is, therefore, manifest that the apex of the radicle of the oak is highly sensitive to contact, and retains its sensitiveness during several days. The movement thus induced was, however, slower than in any of the previous cases, with the exception of that of Aesculus. As with the bean, the terminal growing part, after bending, sometimes straightened itself through the action of geotropism, although the object still remained attached to the tip.

The same remarkable experiment was next tried, as in the case of the bean; namely, little squares of exactly the same size of the card-like sanded paper and of very thin paper (the thicknesses of which have been given under Vicia faba) were attached with shellac on opposite sides (as accurately as could be done) of the tips of 13 radicles, suspended in damp air, at a temperature of 65o - 66o F. The result was striking, for 9 out of these 13 radicles became plainly, and 1 very slightly, curved from the thick paper towards the side bearing the thin paper. In two of these cases the apex became completely hooked after two days; in four cases the deflection from the perpendicular and from the side bearing the thick paper, amounted in from two to four days to angles of 90o, 72o, 60o, and 49o, but in two other cases to only 18o and 15o. It should, however, be stated that in the [page 177] case in which the deflection was 49o, the two squares had accidentally come into contact on one side of the apex, and thus formed a lateral gable; and the deflection was directed in part from this gable and in part from the thick paper. In three cases alone the radicles were not affected by the difference in thickness of the squares of paper attached to their tips, and consequently did not bend away from the side bearing the stiffer paper.

Zea mays: Sensitiveness of the apex of the Radicle to contact.—A large number of trials were made on this plant, as it was the only monocotyledon on which we experimented. An abstract of the results will suffice. In the first place, 22 germinating seeds were pinned to cork-lids without any object being attached to their radicles, some being exposed to a temperature of 65o - 66o F., and others to between 74o and 79o; and none of them became curved, though some were a little inclined to one side. A few were selected, which from having germinated on sand were crooked, but when suspended in damp air the terminal part grew straight downwards. This fact having been ascertained, little squares of the card-like paper were affixed with shellac, on several occasions, to the tips of 68 radicles. Of these the terminal growing part of 39 became within 24 h. conspicuously curved away from the attached squares and from the perpendicular; 13 out of the 39 forming hooks with their points directed towards the zenith, and 8 forming loops. Moreover, 7 other radicles out of the 68, were slightly and two doubtfully deflected from the cards. There remain 20 which were not affected; but 10 of these ought not to be counted; for one was diseased, two had their tips quite surrounded by shellac, and the squares on 7 had slipped so as to stand parallel to the apex, instead of obliquely [page 178] on it. There were therefore only 10 out of the 68 which certainly were not acted on. Some of the radicles which were experimented on were young and short, most of them of moderate length, and two or three exceeded three inches in length. The curvature in the above cases occurred within 24 h., but it was often conspicuous within a much shorter period. For instance, the terminal growing part of one radicle was bent upwards into a rectangle in 8 h. 15 m., and of another in 9 h. On one occasion a hook was formed in 9 h. Six of the radicles in a jar containing nine seeds, which stood on a sand-bath, raised to a temperature varying from 76o to 82o F., became hooked, and a seventh formed a complete loop, when first looked at after 15 hours.

The accompanying figures of four germinating seeds (Fig. 69) show, firstly, a radicle (A) the apex of which has become so much bent away from the attached square as to form a hook. Secondly (B), a hook converted through the continued irritation of the card, aided perhaps by geotropism, into an almost complete circle or loop. The tip in the act of forming a loop generally rubs against the upper part of the radicle, and pushes off the attached square; the loop then contracts or closes, but never disappears; and the apex afterwards grows vertically downwards, being no longer irritated by any attached object. This frequently occurred, and is represented at C. The jar above mentioned with the six hooked radicles and another jar were kept for two additional days, for the sake of observing how the hooks would be modified. Most of them became converted into simple loops, like that figured at C; but in one case the apex did not rub against the upper part of the radicle and thus remove the card; and it consequently made, owing [page 179] to the continued irritation from the card, two complete loops, that is, a helix of two spires; which afterwards became pressed closely together. Then geotropism prevailed and caused the apex to grow perpendicularly downwards. In another case, shown at (D), the apex

Fig. 69. Zea mays: radicles excited to bend away from the little squares of card attached to one side of their tips.

in making a second turn or spire, passed through the first loop, which was at first widely open, and in doing so knocked off the card; it then grew perpendicularly downwards, and thus tied itself into a knot, which soon became tight!

Secondary Radicles of Zea.—A short time after the first radicle has appeared, others protrude from the [page 180] seed, but not laterally from the primary one. Ten of these secondary radicles, which were directed obliquely downwards, were experimented on with very small squares of card attached with shellac to the lower sides of their tips. If therefore the squares acted, the radicles would bend upwards in opposition to gravity. The jar stood (protected from light) on a sand-bath, which varied between 76o and 82o F. After only 5 h. one appeared to be a little deflected from the square, and after 20 h. formed a loop. Four others were considerably curved from the squares after 20 h., and three of them became hooked, with their tips pointing to the zenith,—one after 29 h. and the two others after 44 h. By this latter time a sixth radicle had become bent at a right angle from the side bearing the square. Thus altogether six out of the ten secondary radicles were acted on, four not being affected. There can, therefore, be no doubt that the tips of these secondary radicles are sensitive to slight contact, and that when thus excited they cause the upper part to bend from the touching object; but generally, as it appears, not in so short a time as in the case of the first-formed radicle.

SENSITIVENESS OF THE TIP OF THE RADICLE TO MOIST AIR.

Sachs made the interesting discovery, a few years ago, that the radicles of many seedling plants bend towards an adjoining damp surface.* We shall here endeavour to show that this peculiar form of sensitiveness resides in their tips. The movement is directly the reverse of that excited by the irritants hitherto considered, which cause the growing part of the

* 'Arbeiten des Bot. Institut., in Wrzburg,' vol. i. 1872, p. 209. [page 181]

radicle to bend away from the source of irritation. In our experiments we followed Sachs' plan, and sieves with seeds germinating in damp sawdust were suspended so that the bottom was generally inclined at 40o with the horizon. If the radicles had been acted on solely by geotropism, they would have grown out of the bottom of the sieve perpendicularly downwards; but as they were attracted by the adjoining damp surface they bent towards it and were deflected 50o from the perpendicular. For the sake of ascertaining whether the tip or the whole growing part of the radicle was sensitive to the moist air, a length of from 1 to 2 mm. was coated in a certain number of cases with a mixture of olive-oil and lamp-black. This mixture was made in order to give consistence to the oil, so that a thick layer could be applied, which would exclude, at least to a large extent, the moist air, and would be easily visible. A greater number of experiments than those which were actually tried would have been necessary, had not it been clearly established that the tip of the radicle is the part which is sensitive to various other irritants.

[Phaseolus multiflorus.—Twenty-nine radicles, to which nothing had been done, growing out of a sieve, were observed at the same time with those which had their tips greased, and for an equal length of time. Of the 29, 24 curved themselves so as to come into close contact with the bottom of the sieve. The place of chief curvature was generally at a distance of 5 or 6 mm. from the apex. Eight radicles had their tips greased for a length of 2 mm., and two others for a length of 1 mm.; they were kept at a temperature of 15o - 16o C. After intervals of from 19 h. to 24 h. all were still vertically or almost vertically dependent, for some of them had moved towards the adjoining damp surface by about 10o. They had therefore not been acted on, or only slightly acted on, by the damper air on one side, although the whole upper part was freely exposed. After 48 h. three of these radicles became [page 182] considerably curved towards the sieve; and the absence of curvature in some of the others might perhaps be accounted for by their not having grown very well. But it should be observed that during the first 19 h. to 24 h. all grew well; two of them having increased 2 and 3 mm. in length in 11 h.; five others increased 5 to 8 mm. in 19 h.; and two, which had been at first 4 and 6 mm. in length, increased in 24 h. to 15 and 20 mm.

The tips of 10 radicles, which likewise grew well, were coated with the grease for a length of only 1 mm., and now the result was somewhat different; for of these 4 curved themselves to the sieve in from 21 h. to 24h., whilst 6 did not do so. Five of the latter were observed for an additional day, and now all excepting one became curved to the sieve.

The tips of 5 radicles were cauterised with nitrate of silver, and about 1 mm. in length was thus destroyed. They were observed for periods varying between 11 h. and 24h., and were found to have grown well. One of them had curved until it came into contact with the sieve; another was curving towards it; whilst the remaining three were still vertically dependent. Of 7 not cauterised radicles observed at the same time, all had come into contact with the sieve.

The tips of 11 radicles were protected by moistened gold-beaters' skin, which adheres closely, for a length varying from 1 to 2 mm. After 22 h. to 24 h., 6 of these radicles were clearly bent towards or had come into contact with the sieve; 2 were slightly curved in this direction, and 3 not at all. All had grown well. Of 14 control specimens observed at the same time, all excepting one had closely approached the sieve. It appears from these cases that a cap of goldbeaters' skin checks, though only to a slight degree, the bending of the radicles to an adjoining damp surface. Whether an extremely thin sheet of this substance when moistened allows moisture from the air to pass through it, we do not know. One case indicated that the caps were sometimes more efficient than appears from the above results; for a radicle, which after 23 h. had only slightly approached the sieve, had its cap (1 mm. in length) removed, and during the next 15 h. it curved itself abruptly towards the source of moisture, the chief seat of curvature being at a distance of 2 to 3 mm. from the apex.

Vicia faba.—The tips of 13 radicles were coated with the grease for a length of 2 mm.; and it should be remembered that with these radicles the seat of chief curvature is about [page 183] 4 or 5 mm. from the apex. Four of them were examined after 22h., three after 26 h., and six after 36 h., and none had been attracted towards the damp lower surface of the sieve. In another trial 7 radicles were similarly treated, and 5 of them still pointed perpendicularly downwards after 11 h., whilst 2 were a little curved towards the sieve; by an accident they were not subsequently observed. In both these trials the radicles grew well; 7 of them, which were at first from 4 to 11 mm. in length, were after 11 h. between 7 and 16 mm.; 3 which were at first from 6 to 8 mm. after 26 h. were 11.5 to 18 mm. in length; and lastly, 4 radicles which were at first 5 to 8 mm. after 46 h. were 18 to 23 mm. in length. The control or ungreased radicles were not invariably attracted towards the bottom of the sieve. But on one occasion 12 out of 13, which were observed for periods between 22 h. and 36 h., were thus attracted. On two other occasions taken together, 38 out of 40 were similarly attracted. On another occasion only 7 out of 14 behaved in this manner, but after two more days the proportion of the curved increased to 17 out of 23. On a last occasion only 11 out of 20 were thus attracted. If we add up these numbers, we find that 78 out of 96 of the control specimens curved themselves towards the bottom of the sieve. Of the specimens with greased tips, 2 alone out of the 20 (but 7 of these were not observed for a sufficiently long time) thus curved themselves. We can, therefore, hardly doubt that the tip for a length of 2 mm. is the part which is sensitive to a moist atmosphere, and causes the upper part to bend towards its source.

The tips of 15 radicles were cauterised with nitrate of silver, and they grew as well as those above described with greased tips. After an interval of 24 h., 9 of them were not at all curved towards the bottom of the sieve; 2 were curved towards it at angles of 20o and 12o from their former vertical position, and 4 had come into close contact with it. Thus the destruction of the tip for a length of about 1 mm. prevented the curvature of the greater number of these radicles to the adjoining damp surface. Of 24 control specimens, 23 were bent to the sieve, and on a second occasion 15 out of 16 were similarly curved in a greater or less degree. These control trials are included in those given in the foregoing paragraph.

Avena sativa.—The tips of 13 radicles, which projected between 2 and 4 mm. from the bottom of the sieve, many of [page 184] them not quite perpendicularly downwards, were coated with the black grease for a length of from 1 to 1 mm. The sieves were inclined at 30o with the horizon. The greater number of these radicles were examined after 22 h., and a few after 25 h., and within these intervals they had grown so quickly as to have nearly doubled their lengths. With the ungreased radicles the chief seat of curvature is at a distance of not less than between 3.5 and 5.5 mm., and not more than between 7 and 10 mm. from the apex. Out of the 13 radicles with greased tips, 4 had not moved at all towards the sieve; 6 were deflected towards it and from the perpendicular by angles varying between 10o and 35o; and 3 had come into close contact with it. It appears, therefore, at first sight that greasing the tips of these radicles had checked but little their bending to the adjoining damp surface. But the inspection of the sieves on two occasions produced a widely different impression on the mind; for it was impossible to behold the radicles with the black greased tips projecting from the bottom, and all those with ungreased tips, at least 40 to 50 in number, clinging closely to it, and feel any doubt that the greasing had produced a great effect. On close examination only a single ungreased radicle could be found which had not become curved towards the sieve. It is probable that if the tips had been protected by grease for a length of 2 mm. instead of from 1 to 1 mm., they would not have been affected by the moist air and none would have become curved.