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 circ.u.mstances, 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 rea.s.sume 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 sh.e.l.lac (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 circ.u.mstance, only nine of the squares induced any * "Arbeiten Bot. Inst., Wurzburg," 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 const.i.tution.
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 sh.e.l.lac to the tips were highly efficient in causing the radicles to bend away from them, similar pieces (of about 1/20th
* "Arbeiten Bot. Inst.i.t., Wurzburg," 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 sh.e.l.lac 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. Inst.i.tut., Wurzburg," 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 ill.u.s.tration 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]
a.n.a.logous 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 sh.e.l.lac 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 circ.u.mstances, 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 sh.e.l.lac 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 sh.e.l.lac, 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 a.n.a.logous 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 sh.e.l.lac 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 circ.u.mstances, 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 gla.s.s and rough cinders, affixed with sh.e.l.lac to one side of the apex. Rather large drops of the dissolved sh.e.l.lac 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 a.n.a.logous 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 sh.e.l.lac 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 sh.e.l.lac 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 gla.s.s or squares of the card attached to their tips by means of Canada-balsam or asphalte, which adhered rather better than sh.e.l.lac 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.