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   General considerations—Vicia faba, effects of amputating the tips of the   radicles—Regeneration of the tips—Effects of a short exposure of the tips
  to geotropic action and their subsequent amputation—Effects of amputating
  the tips obliquely—Effects of cauterising the tips—Effects of grease on
  the tips—Pisum sativum, tips of radicles cauterised transversely, and on
  their upper and lower sides—Phaseolus, cauterisation and grease on the
  tips—Gossypium—Cucurbita, tips cauterised transversely, and on their
  upper and lower sides—Zea, tips cauterised—Concluding remarks and summary
  of chapter—Advantages of the sensibility to geotropism being localised in
  the tips of the radicles.
CIESIELSKI states* that when the roots of Pisum, Lens and Vicia were extended horizontally with their tips cut off, they were not acted on by geotropism; but some days afterwards, when a new root-cap and vegetative point had been formed, they bent themselves perpendicularly downwards. He further states that if the tips are cut off, after the roots have been left extended horizontally for some little time, but before they have begun to bend downwards, they may be placed in any position, and yet will bend as if still acted on by geotropism; and this shows that some influence had been already transmitted to the bending part from the tip before it was amputated. Sachs repeated these experiments; he cut off a length of between .05 and 1 mm. (measured from the apex of the
* 'Abwartskrümmung der Wurzel,' Inaug. Dissert. Breslau, 1871, p. 29. [page 524]
vegetative point) of the tips of the radicles of the bean (Vicia faba), and placed them horizontally or vertically in damp air, earth, and water, with the result that they became bowed in all sorts of directions.* He therefore disbelieved in Ciesielski's conclusions. But as we have seen with several plants that the tip of the radicle is sensitive to contact and to other irritants, and that it transmits some influence to the upper growing part causing it to bend, there seemed to us to be no a priori improbability in Ciesielski's statements. We therefore determined to repeat his experiments, and to try others on several species by different methods.
Vicia faba.—Radicles of this plant were extended horizontally either over water or with their lower surfaces just touching it. Their tips had previously been cut off, in a direction as accurately transverse as could be done, to different lengths, measured from the apex of the root-cap, and which will be specified in each case. Light was always excluded. We had previously tried hundreds of unmutilated radicles under similar circumstances, and found that every one that was healthy became plainly geotropic in under 12 h. In the case of four radicles which had their tips cut off for a length of 1.5 mm., new root caps and new vegetative points were re-formed after an interval of 3 days 20 h.; and these when placed horizontally were acted on by geotropism. On some other occasions this regeneration of the tips and reacquired sensitiveness occurred within a somewhat shorter time. Therefore, radicles having their tips amputated should be observed in from 12 to 48 h. after the operation.
Four radicles were extended horizontally with their lower surfaces touching the water, and with their tips cut off for a length of only 0.5 mm.: after 23 h. three of them were still horizontal; after 47 h. one of the three became fairly geotropic; and after 70 h. the other two showed a trace of this action. The fourth radicle was vertically geotropic after 23 h.; but by an
* 'Arbeiten des Bot. Instituts in Würzburg,' Heft. iii. 1873, p. 432. [page 525]
accident the root-cap alone and not the vegetative point was found to have been amputated; so that this case formed no real exception and might have been excluded.
Five radicles were extended horizontally like the last, and had their tips cut off for a length of 1 mm.; after 22-23 h., four of them were still horizontal, and one was slightly geotropic; after 48 h. the latter had become vertical; a second was also somewhat geotropic; two remained approximately horizontal; and the last or fifth had grown in a disordered manner, for it was inclined upwards at an angle of 65o above the horizon.
Fourteen radicles were extended horizontally at a little height over the water with their tips cut off for a length of 1.5 mm.; after 12 h. all were horizontal, whilst five control or standard specimens in the same jar were all bent greatly downwards. After 24 h. several of the amputated radicles remained horizontal, but some showed a trace of geotropism, and one was plainly geotropic, for it was inclined at 40o beneath the horizon.
Seven horizontally extended radicles from which the tips had been cut off for the unusual length of 2 mm. unfortunately were not looked at until 35 h. had elapsed; three were still horizontal, but to our surprise, four were more or less plainly geotropic.
The radicles in the foregoing cases were measured before their tips were amputated, and in the course of 24 h. they had all increased greatly in length; but the measurements are not worth giving. It is of more importance that Sachs found that the rate of growth of the different parts of radicles with amputated tips was the same as with unmutilated ones. Altogether twenty-nine radicles were operated on in the manner above described, and of these only a few showed any geotropic curvature within 24 h.; whereas radicles with unmutilated tips always became, as already stated, much bent down in less than half of this time. The part of the radicle which bends most lies at the distance of from 3 to 6 mm. from the tip, and as the bending part continues to grow after the operation, there does not seem any reason why it should not have been acted on by geotropism, unless its curvature depended on some influence transmitted from the tip. And we have clear evidence of such transmission in Ciesielski's experiments, which we repeated and extended in the following manner.
Beans were embedded in friable peat with the hilum downwards, and after their radicles had grown perpendicularly down for a length of from ? to 1 inch, sixteen were selected which [page 526] were perfectly straight, and these were placed horizontally on the peat, being covered by a thin layer of it. They were thus left for an average period of 1 h. 37 m. The tips were then cut off transversely for a length of 1.5 mm., and immediately afterwards they were embedded vertically in the peat. In this position geotropism would not tend to induce any curvature, but if some influence had already been transmitted from the tip to the part which bends most, we might expect that this part would become curved in the direction in which geotropism had previously acted; for it should be noted that these radicles being now destitute of their sensitive tips, would not be prevented by geotropism from curving in any direction. The result was that of the sixteen vertically embedded radicles, four continued for several days to grow straight downwards, whilst twelve became more or less bowed laterally. In two of the twelve, a trace of curvature was perceptible in 3 h. 30 m., counting from the time when they had first been laid horizontally; and all twelve were plainly bowed in 6 h., and still more plainly in 9 h. In every one of them the curvature was directed towards the side which had been downwards whilst the radicles remained horizontal. The curvature extended for a length of from 5 to, in one instance, 8 mm., measured from the cut-off end. Of the twelve bowed radicles five became permanently bent into a right angle; the other seven were at first much less bent, and their curvature generally decreased after 24 h., but did not wholly disappear. This decrease of curvature would naturally follow, if an exposure of only 1 h. 37 m. to geotropism, served to modify the turgescence of the cells, but not their subsequent growth to the full extent. The five radicles which were rectangularly bent became fixed in this position, and they continued to grow out horizontally in the peat for a length of about 1 inch during from 4 to 6 days. By this time new tips had been formed; and it should be remarked that this regeneration occurred slower in the peat than in water, owing perhaps to the radicles being often looked at and thus disturbed. After the tips had been regenerated, geotropism was able to act on them, so that they now became bowed vertically downwards. An accurate drawing (Fig. 195) is given on the opposite page of one of these five radicles, reduced to half the natural size.
We next tried whether a shorter exposure to geotropism would suffice to produce an after-effect. Seven radicles were extended horizontally for an hour, instead of 1 h. 37 m. as in the [page 527] former trial; and after their tips (1.5 mm. in length) had been amputated, they were placed vertically in damp peat. Of these, three were not in the least affected and continued for days to grow straight downwards. Four showed after 8 h. 30 m. a mere trace of curvature in the direction in which they had been acted on by geotropism; and in this respect they differed much from those which had been exposed for 1 h. 37 m., for many of the latter were plainly curved in 6 h. The curvature of one of these four radicles almost disappeared after 24 h. In the second, the curvature increased during two days and then decreased. the third radicle became permanently bent, so that its terminal part made an angle of about 45o with its original vertical direction. The fourth radicle became horizontal. These two, latter radicles continued during two more days to grow in the peat in the same directions, that is, at an angle of 45o beneath the horizon and horizontally. By the fourth morning new tips had been re-formed, and now geotropism was able to act on them again, and they became bent perpendicularly downwards, exactly as in the case of the five radicles described in the last paragraph and as is shown in (Fig. 195) here given.
Fig. 195. Vicia faba: radicle, rectangularly bent at A, after the amputation of the tip, due to the previous influence of geotropism. L, side of bean which lay on the peat, whilst geotropism acted on the radicle. A, point of chief curvature of the radicle, whilst standing vertically downwards. B, point of chief curvature after the regeneration of the tip, when geotropism again acted. C, regenerated tip.
Lastly, five other radicles were similarly treated, but were exposed to geotropism during only 45 m. After 8 h. 30 m. only one was doubtfully affected; after 24 h. two were just perceptibly curved towards the side which had been acted on by geotropism; after 48 h. the one first mentioned had a radius of curvature of 60 mm. That this curvature was due to the action of geotropism during the horizontal position of the radicle, was shown after 4 days, when a new tip had been re-formed, for it then grew perpendicularly downwards. We learn from this [page 528] case that when the tips are amputated after an exposure to geotropism of only 45 m., though a slight influence is sometimes transmitted to the adjoining part of the radicle, yet this seldom suffices, and then only slowly, to induce even moderately well-pronounced curvature.
In the previously given experiments on 29 horizontally extended radicles with their tips amputated, only one grew irregularly in any marked manner, and this became bowed upwards at an angle of 65o. In Ciesielski's experiments the radicles could not have grown very irregularly, for if they had done so, he could not have spoken confidently of the obliteration of all geotropic action. It is therefore remarkable that Sachs, who experimented on many radicles with their tips amputated, found extremely disordered growth to be the usual result. As horizontally extended radicles with amputated tips are sometimes acted on slightly by geotropism within a short time, and are often acted on plainly after one or two days, we thought that this influence might possibly prevent disordered growth, though it was not able to induce immediate curvature. Therefore 13 radicles, of which 6 had their tips amputated transversely for a length of 1.5 mm., and the other 7 for a length of only 0.5 mm., were suspended vertically in damp air, in which position they would not be affected by geotropism; but they exhibited no great irregularity of growth, whilst observed during 4 to 6 days. We next thought that if care were not taken in cutting off the tips transversely, one side of the stump might be irritated more than the other, either at first or subsequently during the regeneration of the tip, and that this might cause the radicle to bend to one side. It has also been shown in Chapter III. that if a thin slice be cut off one side of the tip of the radicle, this causes the radicle to bend from the sliced side. Accordingly, 30 radicles, with tips amputated for a length of 1.5 mm., were allowed to grow perpendicularly downwards into water. Twenty of them were amputated at an angle of 20o with a line transverse to their longitudinal axes; and such stumps appeared only moderately oblique. The remaining ten radicles were amputated at an angle of about 45o. Under these circumstances no less than 19 out of the 30 became much distorted in the course of 2 or 3 days. Eleven other radicles were similarly treated, excepting that only 1 mm. (including in this and all other cases the root-cap) was amputated; and of these only one grew much, and two others slightly [page 529] distorted; so that this amount of oblique amputation was not sufficient. Out of the above 30 radicles, only one or two showed in the first 24 h. any distortion, but this became plain in the 19 cases on the second day, and still more conspicuous at the close of the third day, by which time new tips had been partially or completely regenerated. When therefore a new tip is reformed on an oblique stump, it probably is developed sooner on one side than on the other: and this in some manner excites the adjoining part to bend to one side. Hence it seems probable that Sachs unintentionally amputated the radicles on which he experimented, not strictly in a transverse direction.
This explanation of the occasional irregular growth of radicles with amputated tips, is supported by the results of cauterising their tips; for often a greater length on one side than on the other was unavoidably injured or killed. It should be remarked that in the following trials the tips were first dried with blotting-paper, and then slightly rubbed with a dry stick of nitrate of silver or lunar caustic. A few touches with the caustic suffice to kill the root-cap and some of the upper layers of cells of the vegetative point. Twenty-seven radicles, some young and very short, others of moderate length, were suspended vertically over water, after being thus cauterised. Of these some entered the water immediately, and others on the second day. The same number of uncauterised radicles of the same age were observed as controls. After an interval of three or four days the contrast in appearance between the cauterised and control specimens was wonderfully great. The controls had grown straight downwards, with the exception of the normal curvature, which we have called Sachs' curvature. Of the 27 cauterised radicles, 15 had become extremely distorted; 6 of them grew upwards and formed hoops, so that their tips sometimes came into contact with the bean above; 5 grew out rectangularly to one side; only a few of the remaining 12 were quite straight, and some of these towards the close of our observations became hooked at their extreme lower ends. Radicles, extended horizontally instead of vertically, with their tips cauterised, also sometimes grew distorted, but not so commonly, as far as we could judge, as those suspended vertically; for this occurred with only 5 out of 19 radicles thus treated.
Instead of cutting off the tips, as in the first set of experiments, we next tried the effects of touching horizontally extended radicles with caustic in the manner just described. But [page 530] some preliminary remarks must first be made. It may be objected that the caustic would injure the radicles and prevent them from bending; but ample evidence was given in Chapter III. that touching the tips of vertically suspended radicles with caustic on one side, does not stop their bending; on the contrary, it causes them to bend from the touched side. We also tried touching both the upper and the lower sides of the tips of some radicles of the bean, extended horizontally in damp friable earth. The tips of three were touched with caustic on their upper sides, and this would aid their geotropic bending; the tips of three were touched on their lower sides, which would tend to counteract the bending downwards; and three were left as controls. After 24 h. an independent observer was asked to pick out of the nine radicles, the two which were most and the two which were least bent; he selected as the latter, two of those which had been touched on their lower sides, and as the most bent, two of those which had been touched on the upper side. Hereafter analogous and more striking experiments with Pisum sativum and Cucurbita ovifera will be given. We may therefore safely conclude that the mere application of caustic to the tip does not prevent the radicles from bending.
In the following experiments, the tips of young horizontally extended radicles were just touched with a stick of dry caustic; and this was held transversely, so that the tip might be cauterised all round as symmetrically as possible. The radicles were then suspended in a closed vessel over water, kept rather cool, viz., 55o - 59o F. This was done because we had found that the tips were more sensitive to contact under a low than under a high temperature; and we thought that the same rule might apply to geotropism. In one exceptional trial, nine radicles (which were rather too old, for they had grown to a length of from 3 to 5 cm.), were extended horizontally in damp friable earth, after their tips had been cauterised and were kept at too high a temperature, viz., of 68o F., or 20o C. The result in consequence was not so striking as in the subsequent cases for although when after 9 h. 40 m. six of them were examined, these did not exhibit any geotropic bending, yet after 24 h., when all nine were examined, only two remained horizontal, two exhibited a trace of geotropism, and five were slightly or moderately geotropic, yet not comparable in degree with the control specimens. Marks had been made on seven of these cauterised radicles at 10 mm. from the tips, which includes [page 531] the whole growing portion; and after the 24 h. this part had a mean length of 37 mm., so that it had increased to more than 3 ? times its original length; but it should be remembered that these beans had been exposed to a rather high temperature.
Nineteen young radicles with cauterised tips were extended at different times horizontally over water. In every trial an equal number of control specimens were observed. In the first trial, the tips of three radicles were lightly touched with the caustic for 6 or 7 seconds, which was a longer application than usual. After 23 h. 30 m. (temp. 55o - 56o F.) these three radicles,
Fig. 196. Vicia faba: state of radicles which had been extended horizontally for 23 h. 30 m.; A, B, C, tips touched with caustic; D, E, F, tips uncauterised. Lengths of radicles reduced to one-half scale, but by an accident the beans themselves not reduced in the same degree.
A, B, C (Fig. 196), were still horizontal, whilst the three control specimens had become within 8 h. slightly geotropic, and strongly so (D, E, F) in 23 h. 30 m. A dot had been made on all six radicles at 10 mm. from their tips, when first placed horizontally. After the 23 h. 30 m. this terminal part, originally 10 mm. in length, had increased in the cauterised specimens to a mean length of 17.3 mm., and to 15.7 mm. in the control radicles, as shown in the figures by the unbroken transverse line; the dotted line being at 10 mm. from the apex. The control or uncauterised radicles, therefore, had actually grown less [page 532] than the cauterised; but this no doubt was accidental, for radicles of different ages grow at different rates, and the growth of different individuals is likewise affected by unknown causes. The state of the tips of these three radicles, which had been cauterised for a rather longer time than usual, was as follows: the blackened apex, or the part which had been actually touched by the caustic, was succeeded by a yellowish zone, due probably to the absorption of some of the caustic; in A, both zones together were 1.1 mm. in length, and 1.4 mm. in diameter at the base of the yellowish zone; in B, the length of both was only 0.7 mm., and the diameter 0.7 mm.; in C, the length was 0.8 mm., and the diameter 1.2 mm.
Three other radicles, the tips of which had been touched with caustic curing 2 or 3 seconds, remained (temp. 58o - 59o F.) horizontal for 23 h.; the control radicles having, of course, become geotropic within this time. The terminal growing part, 10 mm. in length, of the cauterised radicles had increased in this interval to a mean length of 24.5 mm., and of the controls to a mean of 26 mm. A section of one of the cauterised tips showed that the blackened part was 0.5 mm. in length, of which 0.2 mm. extended into the vegetative point; and a faint discoloration could be detected even to 1.6 mm. from the apex of the root-cap.
In another lot of six radicles (temp. 55o - 57o F.) the three control specimens were plainly geotropic in 8 ? h.; and after 24 h. the mean length of their terminal part had increased from 10 mm. to 21 mm. When the caustic was applied to the three cauterised specimens, it was held quite motionless during 5 seconds, and the result was that the black marks were extremely minute. Therefore, caustic was again applied, after 8 ? h., during which time no geotropic action had occurred. When the specimens were re-examined after an additional interval of 15 ? h., one was horizontal and the other two showed, to our surprise, a trace of geotropism which in one of them soon afterwards became strongly marked; but in this latter specimen the discoloured tip was only 2/3 mm. in length. The growing part of these three radicles increased in 24 h. from 10 mm. to an average of 16.5 mm.
It would be superfluous to describe in detail the behaviour of the 10 remaining cauterised radicles. The corresponding control specimens all became geotropic in 8 h. Of the cauterised, 6 were first looked at after 8 h., and one alone showed a trace [page 533] of geotropism; 4 were first looked at after 14 h., and one alone of these was slightly geotropic. After 23 - 24h., 5 of the 10 were still horizontal, 4 slightly, and 1 decidedly, geotropic. After 48 h. some of them became strongly geotropic. The cauterised radicles increased greatly in length, but the measurements are not worth giving.
As five of the last-mentioned cauterised radicles had become in 24 h. somewhat geotropic, these (together with three which were still horizontal) had their positions reversed, so that their tips were now a little upturned, and they were again touched with caustic. After 24 h. they showed no trace of geotropism; whereas the eight corresponding control specimens, which had likewise been reversed, in which position the tips of several pointed to the zenith, all became geotropic; some having passed in the 24 h. through an angle of 180o, others through about 135o, and others through only 90o. The eight radicles, which had been twice cauterised, were observed for an additional day (i.e. for 48 h. after being reversed), and they still showed no signs of geotropism. Nevertheless, they continued to grow rapidly; four were measured 24 h. after being reversed, and they had in this time increased in length between 8 and 11 mm.; the other four were measured 48 h. after being reversed, and these had increased by 20, 18, 23, and 28 mm.
In coming to a conclusion with respect to the effects of cauterising the tips of these radicles, we should bear in mind, firstly, that horizontally extended control radicles were always acted on by geotropism, and became somewhat bowed downwards in 8 or 9 h.; secondly, that the chief seat of the curvature lies at a distance of from 3 to 6 mm. from the tip; thirdly, that the tip was discoloured by the caustic rarely for more than 1 mm. in length; fourthly, that the greater number of the cauterised radicles, although subjected to the full influence of geotropism during the whole time, remained horizontal for 24 h., and some for twice as long; and that those which did become bowed were so only in a slight degree; fifthly, that the cauterised radicles continued to grow almost, and sometimes quite, as well as the uninjured ones along the part which bends most. And lastly, that a touch on the tip with caustic, if on one side, far from preventing curvature, actually induces it. Bearing all these facts in mind, we must infer that under normal conditions the geotropic curvature of the root is du............
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