The Power of Movement in Plants

Amphicarpea monoica.—This plant produces long thin shoots, which twine round a support and of course circumnutate. Early in the summer shorter shoots are produced from the lower parts of the plant, which grow perpendicularly downwards and penetrate the ground. One of these, terminating in a minute bud, was observed to bury itself in sand to a depth of 0-2 inch in 24h. It was lifted up and fixed in an inclined position about 25° beneath the horizon, being feebly illuminated from above. In this position it described two vertical ellipses in 24h.; but on the following day, when brought into the house, it circumnutated only a very little round the same spot. Other branches were seen to penetrate the ground, and were after- wards found running like roots beneath the surface for a length of nearly two inches, and they had grown thick. One of these, after thus running, had emerged into the air. How far circum- nutation aids these delicate branches in entering the ground we do not know; but the reflexed hairs with which they are clothed will assist in the work. This plant produces pods in the air, and others beneath the ground; which differ greatly in appear- ance. Asa Gray says* that it is the imperfect flowers on the creeping branches near the base of the plant which produce the subterranean pods; these flowers, therefore, must bury them- selves like those of Arachis. But it may be suspected that the branches which were seen by us to penetrate the ground also produce subterranean flowers and pods.

Besides geotropism and apogeotropism, there is, according t) Frank, an allied form of movement, namely, “ transverse-geotropism,” or diageotropism, as we may call it for the sake of matching our other terms. Under the influence of gravitation certain parts are excited to place themselves more or less transversely to the line of its action.* We made no observations on this subject, and will here only re- mark that the position of the secondary radicles of various plants, which extend horizontally or are a little inclined downwards, would probably be con- sidered by Frank as due to transverse-geotropism. As it has been shown in Chap. I. that the secondary radicles of Cucurbita made serpentine tracks on a smoked glass-plate, they clearly circumnutated, and there can hardly be a doubt that this holds good with other secondary radicles. It seems there- fore highly probable that they place themselves in their diageotropic position by means of modified circumnutation.

Finally, we may conclude that the three kinds of movement which have now been described and which are excited by gravitation, consist of modified circum- nutation. Different parts or organs on the same plant, and the same part in different species, are thus excited to act in a widely different manner. We can see no yeason why the attraction of gravity should directly modify the state of turgescence and subsequent growth of one part on the upper side and of another part on the lower side. Weare therefore led to infer that both geotropic, apogeotropic, and diageotropic movements, the purpose of which we can generally understand,

* Elfving has lately described excellent instance of such move- ( Arbeiten des Bot. Instituts in ments in the :hizomes of certain Wiirzburg.’ B. ii. 1880, p.489, an _ plants, have been acquired for the advantage of the plant by the modification of the ever-present movement of circumnutation, This, however, implies that gravi- tation produces some effect on the young tissues sufficient to serve as a guide to the plant. LovaLisED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFrrFects.

General considerations—Vicia faba, effects of amputating the tips of the radicles Regeneration uf 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 greiuse 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, t:ps 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 them- selves 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

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 direc- tions.* He therefore disbelieved in Ciesielski’s con- clusions. 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 fabt.—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 accu- rately 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 circeum- stances, and found that every one that was healthy becane plainly geotropic in under 12h. In the case of four radicles which had their tips cut off for a length of 15 mm., new r.ot- 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 some- what 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 tonching 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 23h.; but by an 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 heen excluded.

Five radicles were extended horizontally like the last, and had their tips cut off for a length of L mm.; after 22-¥3 h., four of them were still horiz ntal, and one was slightly geotropic; after 48 h. the latter had become vertical; a second was also somes hat geotropic; two remained approximately horizontal ; and the last or tifth had grown in a disordered manner, for it was inclined upwards at an angle of 65° above the horizon. Fourteen racicles were extended horizontally at a little height over the water with their tips cut off for a length of 1-5 mm.; after 12h. all were horizontal, whilst five control or standard specimens in the same jar were all bent greatly downwards. After 24h. several of the amputated radicles remained hori- zontal, but some showed a trace of geotropism, and one was plainly geotropic, for it was inclined at 40° 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 witb amputated tips was the same as with unmutilated ones. Alto- gether 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 8 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 down- wards, and after their radicles had grown perpendicularly down for a length of from 4 to 1 inch, sixteen were selected which were perfectly straight, and these were placed horizontally ou the peat, being covered by a thin layer of it. They were thus left for an average periodof 1h. 37m. The tips were then cut off transversely for a length of 1-5 mm., and immediately after- wards tliey were embedded vertically in the peat. In ‘his 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 9h. 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 in- stance, 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 k., but did not wholly disappear. This decrcase of curvature would naturally follow, if an ex- posure of only 1 h. 87 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 becn regenerated, geotropism was able to act on them, so that they now lecame bowed vertically downwards. An accurate draw- ing (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-cffect. Seven radicles were extended horizontally for an hour, instead of 1h. 87m as in the 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 8h 30m. 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 lh. 37 m., for many of the latter were plainly curved in 6h. The curvature of one of these four radicles almost disappeared after 24 h. In the second, the cur- vature increased during two days and then decreased. The third radicle became permanently bent, so that its terminal part made an angle of about 45° 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 45° be- neath the horizon and horizon- tally. By the fourth morning new tips had been re-formed, and now

Vicia faba; radicle, rectangularly bent at A, after the ampu' ation of the tip, due to the previous influence of geotropi-m. L, side of bean which lay on the peat, 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 whilst geotropism acted on the radicle. A, point of chief cur- vature of the radicle, whilst standing vertically downwards. B, point of chief curvature after theregeneration of the tip, when geotropism again actel. C, re- generated tip.

Lastly, five other radicles were similarly treated, but were ex- posed to geotropism during only 45m. After 8 h. 30 m. only one was doubtfully affected; after 24 h. two were just per- ceptibly curved towards the side which had bee.: acted on by geotropism; after 48 h. the one first mentioned had a radius of curvature of 60 wm. That this curvature was duc to the ation of geotropism during the horizontal position of the radicle, waa shown after 4 days, when a new tip had l.en reformed, fur it then grew perpendicularly downwards. We learn from thie

cace that when the tips are amputated after an exposure to gco- tropism of only 45 m., though a slight influence is sometimes transmitted to the adjoining part of the radicle, yet this seldom sitffices, and then only slowly, to induce even moderately well- pronounced curvature. In the previously given experiments on 29 horizontally ex- tended radl.cles with their tips amputated, only one grew irre- gularly in any marked manner, and this became bowed upwards at an angle of 65°. In Ciesielski’s experiments the radicles could not have grown very irregularly, for if they had done so, he conld 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 some- times 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. There- fore 13 radicles, of which 6 had their tips amputated trans- versely for a length of 1:5 mm., and the other 7 for a length of only 05 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 duving 4 to 6 days. We next thought that if care were uot taken in cutting off the tips transversely, one side of the stump might be irritated more than the other, either at first or sub- sequently 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 15 mm., were allowed to grow perpendicularly downwards into water. Twenty of them were amputated at an angle of 20° 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 45°. Under these cireumstances no less than 19 out of the 30 became much distorted in the course of 2 or 8 days. Eleven other radicles were similarly treated, excepting that only 1 mm. (including in this and all other cascs the root-cap) was ampu- tated; and of these only one grew much and two others slightly 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 re- formed 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. Henceit seems probable that Sachs unintentionally amputated the radicles on which he expcrimented, not strictly iu 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 re- marked 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 appcarance 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 cauteri ed radicles, 15 had become extremely distorted; 6 of them grew upwards and formed hoops, so that their tips some- times 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 cl.se 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 com- monly, as far as we could judge, as those suspended vertically; for this oceurred with only 5 out of 19 radicles thus treated.

’ Instead of cutting off the tips, as in the first set of experi- ments, we next tried the effects of touching horizontally ex- tended radicles with caustic in the manner just described. But some preliminary remarks must first be made. It may be ob- jected that the caustic would injure the radicles and prevent them from bending; but ample evidence was given in Chapter ITI. that touching the tips of vertically suspended radicles with caustic on one side, does not stop their benling; 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. Tbe 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 24h. 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, tne 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 cau- terised all round as symmetrically as possible. The radicles were then suspended in a closed vessel over water, kept rather cool, viz., 55°-59° F. This was done because we had found that the tips were more sensitive to contact under ae 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 taey 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 68° F., or 20°C. The result in consequence was not so striking as in the subsequent eases; for although when after 9 h. 40 m. six of them were exumined, these did not exhibit any geotropic bending, yet after 24h., when all nine were examined, only two remained hori- zontal, two exhibited a trace of geotropism, and five were slightly or mocterately geotropic, yet not comparable in degree with the control specimens. Marks had been made on seven of these cauterised radicles at 10mm. from the tips, which includes

the whole growing portion; and after the 24h. this pact had a mean length of 87 mm., so that it had increased to more than 33 times its original length; but it should be remembered that these beans had been exposed tu 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. {n the first trial, the tips of three radicles were lightly touched with the caustic for 6 or 7 seconds, which was a longer application thau usual. After 23h. 30 m. (temp. 55°-56° F.) these three radicies,

Vicia faba. state of radieles which had been extended horizontally for 23h. 30m.: A, B. C, tips touched with caustic; D, E, F, tips uncaute- rised. 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 geotr pic, and strongly so (\D, E, F) in 23 b. 30m. A dot had been made on all six radicles at 10 mm. from their tips, when first placed horizontally. After the 23 h. 30m. 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 ccn- trol or uncauierised radicles, therefore, had actually grown less

than the cauterised; but this no doubt was accidental, for radicles of different ages grow at different rites, and the growth of different individuals is hkewise affected by unknown causcvs, 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 during 2 or 3 seconds, remained (temp. 58°-59° 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 toa mean of 26mm. A section of one of the cauterised tips showed that the blackened part was 0°5 mm. in length, of which 0°2mm. extended into the vegetative point; and a faint dis- coloration could be detected even to 16mm. from the apex of the root-cap.

In another lot of six radivles (temp. 55°-57° 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. to21mm. 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 ex- tremely minute. Therefore, caustic was again applied, after 8: h., during which time no geotropie action had occurred. When the specimens were re-examined after an additional interval of 153 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 2mm. in length. The growing part of these three radicles increased in 24h. 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 herame geotropic in 8h. Of the cauterised, 6 were first looked at after 8h., and one alone showed a trace of geotropism ; 4 were first looked at after 14h., and one alone ot these was slightly geotropic. After 23-24h.,5 of the 10 were still horizontal, 4 slightly, and 1 decidedly, geotropic. After 48h. 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 24h. they showed no trace of geotropism ; whereas the eight corresponding control specimens, which had like- wise been reversed, in which position the tips of several pointed to the zenith, all became geotropic; some having passed iu the 24h. through an angle of 180°, others through about 185°, and others through only 90°. The eight radicles, which had been twice cauterised, were observed for an additional day (ie. for 48 h. after being reversed), and they still showed no signs of geotro- pism. Nevertheless, they continued to gr.w rapidly ; four were measured 24h. after being reversed, and they had in this time increased in length betweon 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 cauter- ising the tips of these radicles, we should bear in mind, firstly, that horizontally extended control radicles were always acted on by geotropism, and becaine somewhat bowed down- wards in 8 or 9h.; secondly, that the chief seat of the curvature lies at a distance of from 8 to 6mm. from the tip; thirdly, that the tip was discoloured by the caustic rarely for more than 1mm. in length; fourthly, that the greater number of the cau- terised radicles, although subjected to the full influence of geotropism during the whole time, remained horizontal for 2+ h., and some for twice as long; and that those which did become bowed were so only in a slight degree; fifthly, that the cau- terised radicles continued to grow almost, and sometimes quite, as well as the uninjurcd 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 duc to an influence trans- mitted from the apex to the adjoining part where the bending

takes place; and that when the tip of the root is cauterised it ig anable to originate tbe stimulus necessary to produce geotropio curvature. As we had observed that grease was highly injurious to some plants, we determined to try its effects on radicles. When the cotyledons of Phalaris and Avena were covered with grease along one side, the growth of this side was quite stopped or greatly checked, and as the opposite side continued to grow, the cotyledons thus treated became bowed towards the greased side, This same matter quickly killed the delicate hypocotyls and young leaves of certain plants. The grease which we employed was made by mixing lamp-black and olive oil to such a con- sistence that it could be laid on in a thick layer. The tips of five radicles of the bean were coated with it for a length of 3 mm., and to our surprise this part increased in length in 23 h. to 7-1 mm.; the thick layer of grease being curiously drawn out. It thus could not have checked much, if at all, the growth of the terminal part of the radicle. With respect to geotropism, the tips of seven horizontally extended radicles were coated for a length of 2 mm., and after 24 h. no clear difference could be perceived between their downward curvature and that of an equal number of control specimens. The tips of 33 other radicles were coated on different occasions for a length of 3 inm.; and they were compared with the controls after 8h., 24h., and 48h. On one occasion, after 24h., there was very little difference in curviture between the greased and control specimens; but generally the difference was unmistakable, those with greased tips being considerably less curved downwards. The whole growing part (the greased tips included) of six of these radicles was measured and was found to have increased in 23h. from 10 mm. to a mean length of 17:7 mm.; whilst the corresponding part of the contro's had increased to 20°38 mm.