The Power of Movement in Plants

It appears there- fore, that although the tip itself, when greased, continues to grow, yet the growth of the whole radicle is somewhat checked, and that the geotropic curvature of the upper part, which was free from grease, was in most cases considerably lessened. Pisun sativum.—Five radicles, extended horizontally over water, had their tips lightly touched two or three times with dry caustic. These tips were measured in two cases, and found to be blackened for a length of only half a millimeter. Five other radicles were left as controls. The part which is most bowed through geotropism lies at a distance of several millimeters from

the apex. After 24h., and again after 32h. from the commence- ment, four of the cauterised radicles were still horizontal, but one was plainly geotropic, being inclined at 45° beneath the horizon. The five controls were somewhat geotropic after 7 h. 20 m., and after 24h. were all strongly geotropic; being inclined at the following angles beneath the horizon, yiz., 59°, 60°, 65°, 57°, and 43°. The length of the radicles was not measured in either set, but it was manifest that the cauterised radicles had grown greatly.

The following case proves that the action of the caustic by itself does not prevent the curvature of the radicle. Ten radicles were extended horizontally on and beneath a layer of damp friable peat-earth; and before being extended their tips were touched with dry caustic on the upper side. Ten other radicles similarly placed were touched on the lower side; and this would tend to make them bend from the cauterised side; and therefore, as now placed, upwards, or in opposition to geotropism. Lastly, ten uncauterised radicles were extended horizontally as controls. After 24 h. all the latter were geotropic; and the ten with their tips cauterised on the upper side were equally geotropic; and we believe that they became curved downwards before the con- trols. The ten which had been cauterised on the lower side presented a widely different appearance: No. 1, however, was perpendicularly geotropic, but this was no real exception, for on examination under the microscope, there was no vestige of a coloured mark on the tip, and it was evident that by a mistake it had not been touched with the caustic. No. 2 was plainly geotropic, being inclined at about 45° beneath the horizon; No.3 was slightly, and No.4 only just perceptibly geotropic; Nos. 5 and 6 were strictly horizontal; and the four remaining ones were bowed upwards, in opposition to geotropism. In these four cases the radius of the upward curvatures (according to Sachs’ cyclometer) was 5mm., 10 mm., 80 mm.,and 70mm. ‘This cur- vature was distinct long betore the 24h. had elapsed, namely, after 8h. 45m. from the time when the lower sides of the tips were touched with the caustic.

Phaseolus muitiflorus.—EHight radicles, serving as controls, were extended horizontally, some in damp friable peat and some in damp air. They ail became (temp. 20°-21° C.) plainly geo- tropic in 8h. 30 m., for they then stood at an average angle of 63° beneath the horizon. A rather greater length of the radicle is bowed downwards by geotropism than in the case of Victu fuba that is to say, rather more than 6mm. as measured from the apex of the root-cap. Nine other radicles were similarly extended, three in damp peat and six in damp air, and dry caustic was held transversely to their tips during 4 or 5 seconds. Three of their tips were afterwards examined: in (1) alength of 0°68 mm. was discoloured, of which the basal 0°186 mm. was yellow, the apical part being black; in (2) the discoloration was 0°65 mm. in length, of which the basal 0:04. mm. was yellow ; in (8) the dis- voloration was 0°6 mm. in length, of which the hasal 0:18 mm. was yellow. Therefore less than 1 mm. was affected by the caustic, but this sufficed almost wholly to prevent geotropic action; for after 24 h. one alone of the nine cauterised radicles became slightly geotropic, being now inclined at 10° beneath the horizon; the eight others remained horizontal, though one was curved a little laterally.

The terminal part (10 mm. in length) of the six cauterised radicles in the damp air, had more than doubled in length in the 24 h., for this part was now on an average 20°7 mm. long. The increase in length within the same time was greater in the control specimens, for the terminal part had grown on an average from 10 mm. to 26-6 mm. But as the cauterised radicles had more than doubled their length in the 24 h., it is manifest that they had not been seriously injured by the caustic. We may here add that when experimenting on the effects of touching one side of the tip with caustic, too much was applied at first, and the whole tip (but we believe not more than 1 mm. in length) of six horizontally extended radicles wag killed, and these continued for two or three days to grow out horizontally.

Many trials were made, by coating the tips of horizontally extended radicles with the before described thick grease. The geotropic curvature of 12 radicles, which were thus coated for a length of 2 mm., was delayed during the first 8 or 9 h., but after 24 h. was nearly as great as that of the control speci- mens. The tips of nine radicles were coated for a length of 8 mm., and after 7 h. 10 m. these stood at an average angle of 30° beneath the horizon, whilst the controls stood at an average of 51°. After 24 h. the two lots differed but little in their degree of curvature. In some other trials, however, there was a fairly well-marked difference after 24 h. between those with greased tips and the controls. The terminal part of eight control speci- mens increased in 24 h. from 10 mm. to a mean length of

24°3 mm., whilst the mean increase of those with greased tips was 20°7 mm. The grease, therefore, slightly checked the growth of the terminal part, but this part was not much injured; for several radicles which had been greased for a length of 2 mm. continued to grow during scven days, and were then only a little shorter than the controls. The appearance presented by these radicles after the seven days was very curious, for the black grease had been drawn out into the finest longitudinal strie, with dots and reticulations, which covered their surfaces for a length of from 26 to 44 mm., or of 1 to 1:7 inch, We may therefore conclude that grease on the tips of the radicles of this Phaseolus somewhat delays and lessens the geotropic curvature of the part which ought to bend most.

Gossypium herbaceum.—The ravlicles of this plant bend, through the action of geotropism, for a length of about 6 mm. Five radicles, placed horizontally in damp air, had their tips touched with caustic, and the discoloration extended for a length of from 2 tol mm. They showed, after 7 h. 45 m. and again after 23 h., not a trace of geotropism; yet the terminal portion, 9 mm. in length, had increased on an average to 15°9mm. Six control radicles, after 7h. 45 m., were all plainly geotropic, two of them being vertically dependent, and after 23 h. all were vertical, or nearly so.

Cucurbita ovifera.—A large number of trials proved almost useless, from the three following causes: Firstly, the tips of radicles which have grown somewhat old are only feebly geo- tropic if kept in damp air; nor did we succeed well in our experiments, until the germinating seeds were placed in peat and kept at a rather high temperature. Secondly, the hypocotyls of the seeds which were pinned to the lids of the jars gradually became arched; and, as the cotyledons were fixed, the movement of the bypocotyl affected the position of the radicle, and caused confusion. Thirdly, the point of the radicle is so fine that it is difficult not to cauterise it either too much or too little. But we managed generally to overcome this latter difficulty, as the following experiments show, which are given to prove that a touch with caustic on one side of the tip does not prevent the upper part of the radicle from bending. Ten radicles were laid horizontally beneath and on damp friable peat, and their tips were touched with caustic on the upper side. After 8h. all were plainly geotropic, three of them rectangularly; after 19 h.

all were strongly geotropic, most of them pointing perpen- dicularly downwards. Ten other radicles, similarly placed, had their tips touched with caustic on the lower side; after 8 h. three were slightly geotropic, but not nearly so much so as the least geotropic of the foregoing specimens; four remained hori- zontal; and three were curved upwards in opposition to geo- tropism. After 19 h. the three which were slightly geotropic had become strongly so. Of the four horizontal radicles, one alone showed a trace of geotropism; of the three up-curved radicles, one retained this curvature, and the other two had become horizontal.

The radicles of this plant, as already remarked, do not succeed well in damp air, but the result of one trial may be briefly given. Nine young radicles between 3 and ‘5 inch in length, with their tips cauterised and blackened for a length never exceeding 4 mm., together with eight control specimens, were extended horizontally in damp air. After an interval of only 4h. 10 m. all the controls were slightly geotropic, whilst not one of the cauterised specimens exhibited a trace of this action. After 8 h. 85 m., there was the same difference between the two sets, but rather more strongly marked. By this time both sets had increased greatly in length. The controls, however, never became much more curved downwards; and after 24h. there was no great difference between the two sets in their degree of curvature.

Eight young radicles of nearly equal length (average ‘36 inch) were placed beneath and on peat-earth, and were exposed to a temp. of 75°-76° F. Their tips had been touched transversely with caustic, and five of them were blackened for a length of about 0°56 mm., whilst the other three were only just visibly dis- coloured. In the same box there were 15 control radicles, mostly about ‘36 inch in length, but some rather longer and older, and therefore less sensitive. After 5 h., the 15 control radicles were all more or less geotropic: after 9 h., eight of them were bent down beneath the horizon at various angles between 45° and 90°, the remaining seven being only slightly geotropic: after 25 h. all were rectangularly geotropic. The state of the eight cauterised radicles after the same intervals of time was as follows: after 5 h. one alone was slightly geotropic, and this was one with the tip only a very little discoloured: after 9 h. the one just mentioned was rectangularly geotropic, and two others were slightly so, and these were the three which had been scarcely

affected by the caustic; the other five were still strictly hori- zontal. After 24 h. 40 m. the three with only slightly discoloured tips were bent down rectangularly; the other five were not in the least affected, but several of them had grown rather tor- tuously, though still in a horizontal plane. The eight cauterised radicles which had at first a mean length of 86 inch, after 9h. had increased to a mean length of ‘79 inch; and after 24 h. 40 m. to the extraordinary mean length of 2 inches. There was no plain difference in length between the five well cau- terised radicles which remained horizontal, and the three with slightly cauterised tips which had become abruptly bent down. A few of the control radicles were measured after 25 h., and they were on an average only a little longer than the cauterised, viz., 2°19 inches. We thus see that killing the extreme tip of the radicle of this plant for a length of about 0°5 mm., though it stops the geotropic bending of the upper part, hardly interferes with the growth of the whole radicle.

In the same box with the 15 control specimens, the rapid geo- tropic bending and growth of which have just been described, there were six radicles, about ‘6 inch in length, extended hori- zontally, from which the tips had been cut off in a transverse direction for a length of barely 1 mm. These radicles were examined after 9 h. and again after 24 h. 40 m,, and they all remained horizontal. They had not become nearly so tortuous as those above described which had been cauterised. The radicles with their tips cut off had grown in the 24 h. 40 m. as much, judging by the eye, as the cauterised specimens.

Zea mays.—The tips of several radicles, extended horizontally in damp air, were dried with blotting-paper and then touched in the first trial during 2 or 3 seconds with dry caustic; but this was too long a contact, for the tips were blackened for a length of rather above 1mm. They showed no signs of geo- tropism after an interval of 9 h., and were then thrown away. In a second trial the tips of three radicles were touched for a shorter time, and were blackened for a length of from 0°5 to 0°75 mm.: they all remained horizontal for 4h., but after 8 h. 30 m. one of them, in which the blackened tip was only 0°5 mm. in length, was inclined at 21° beneath the horizon. Six con- trol radicles all became slightly geotropic in 4 h., and strongly so after 8h. 80 m., with the chief seat of curvature generally between 6 or 7 mm. from the apex. In the cauterised specimens, the terminal growing part, 10 mm. in length, increased during

the 8h. 30 m. to a mean length of 18 mm.; and in the controls In a third trial the tips of five radicles (exposed to a temp. of 70°-71°) were touched with the caustic only once and very slightly ; they were afterwards examined under the microscope, and the part which was in any way discoloured was on an average ‘76 mm. in length. After 4h. 10 m. none were bent; after 5h. 45 m., and again after 23 h. 30 m., they still remained horizontal, excepting one which was now inclined 20° beneath the horizon. The terminal part, 10 mm. in length, had in- creased greatly in length during the 23 h. 30 m., viz., to an average of 26 mm. Four control radicles became slightly geo- tropic after the 4 h. 10 m., and plainly so after the 5 h. 45 m. Their mean length after the 23 h. 30 m. had increased from 10 mm. to31mm. Therefore a slight cauterisation of the tip checks slightly the growth of the whole radicle, and manifestly stops the bending of that part which ought to bend most under the influence of geotropism and which still continues to increase greatly in length.

Concluding Remarks.—Abundant evidence has now been given, showing that witn various plants the tip of the radicle is alone sensitive to geotropism; and that when thus excited, it causes the adjoining parts to bend. The exact length of the sensitive part seems to be somewhat variable, depending in part on the age of the radicle; but the destruction of a length of from less than 1 to 15 mm, (about 25th of an inch), in the several species observed, generally sufficed to prevent any part of the radicle from bending within 24 h., or even for a longer period. The fact of the tip alone being sensitive is so remarkable a fact, that we will here give a brief summary of the foregoing experiments. The tips were cut off 29 horizontally extended radicles of Vicia faba, and with a few exceptions they did not become geotropic in 22 or 23 h., whilst unmutilated radicles were always bowed downwards in 8 or 9h. It should be borne in mind that the mere act of cutting

off the tip of a horizontally extended radicle does not prevent the adjoining parts from bending, if the tip has been previously exposed for an hour or two to the influence of geotropism. The tip after amputation is sometimes completely regenerated in three days; and it is possible that it may be able to transmit an impulse to the adjoining parts before its complete regeneration. The tips of six radicles of Cucurbita ovifera were amputated like those of Vieta faba ; and these radicles showed no signs of geotropism in 24h. ; whereas the control specimens were slightly affected in 5 h., and strongly in 9 h.

With plants belonging to six genera, the tips of the radicles were touched transversely with dry caustic ; and the injury thus caused rarely extended fora greater length than 1 mm., and sometimes to a less distance, as judged by even the faintest discoloration. We thought that this would be a better method of destroying the vegetative point than cutting it off ; for we knew, from many previous experiments and from some given in the present chapter, that a touch with caustic on one side of the apex, far from preventing the adjoining part from bending, caused it to bend. In all the following cases, radicles with uncauterised tips were observed at the same time and under similar circum- stances, and they became, in almost every instance, plainly bowed downwards in one-half or one-third of the time during which the cauterised specimens were observed. With Viera faba 19 radicles were cau- terised; 12 remained horizontal during 23-24 h.; 6 became slightly and 1 strongly geotropic. Eight of these radicles were afterwards reversed, and again touched with caustic, and none of them became geo- tropic in 24 h., whilst the reversed control specimens became strongly bowed downwards within this time.

With Piswm sativum, five radicles had their tips touched with caustic, and after 32 h. four were still horizontal. The control specimens were slightly geotropic in 7h. 20 m., and strongly so in 24h. The tips of 9 other radicles at this plant were touched only on the lower side, and 6 of them remained horizontal for 24 h., or were upturned in opposition to geotropism; 2 were slightly, and 1 plainly geotropic. With Phaseolus multiflorus, 15 radicles were cauterised, and 8 re- mained horizontal for 24h.; whereas all the controls were plainly geotropic in 8h. 30m. Of 5 cauterised radicles of Gossypium herbaceum, 4 remained horizontai for 23 h. and 1 became slightly geotropic; 6 controi radicles were distinctly geotropic in 7 h.45 m. Five radicles of Cucurbita ovifera remained horizontal in peat-earth during 25 h., and 9 remained so in damp air during 8} h.; whilst the controls became slightly geotropic in 4h.10m. The tips of 10 radicals of this plant were touched on their lower sides, and 6 of them remained horizontal or were upturned after 19 h., 1 being slightly and 3 strongly geotropic.

Lastly, the tips of several radicles of Vicca, faba and Phaseolus multiflorus were thickly coated with grease fora length of 83 mm. This matter, which is highly injurious to most plants, did not kill or stop the growth of the tips, and only slightly lessened the rate of growth of the whole radicle ; but it generally delayed a little the geotropic bending of the upper part. The several foregoing cases would tell us nothing, if the tip itself was the part which became most bent; but we know that it is a part distant from the tip by some millimeters which grows quickest, and which, under the influence. of geotropism, bends most. We have no reason to suppose that this part is injured by the death or injury of the tip; and it is certain

that after the tip has been destroyed this part goes on growing at such a rate, that its length was often doubled inaday. We have also seen that the destruction of the tip does not prevent the adjoining part from bending, i1 this part has already received some influence from the tip. As with horizontally extended radicles, of which the tip has been cut off or destroyed, the part which ought to bend most remains motionless for many hours or days, although exposed at right angles to the full influence of geotropism, we must conclude that the tip alone is sensitive to this power, and trans- mits some influence or stimulus to the adjoining parts, causing them to bend. We have direct evidence of such transmission ; for when a radicle was left extended horizontally for an hour or an hour and a half, by which time the supposed influence will have travelled a little distance from the tip, and the tip was then cut off, the radicle afterwards became bent, although placed perpendicularly. The terminal portions of several radicles thus treated continued for some time to grow in the direction of their newly-acquired curva- ture; for as they were destitute of tips, they were no longer acted on by geotropism. But after three or four days when new vegetative points were formed, the radicles were again acted on by geotropism, and now they curved themselves perpendicularly downwards. To see anything of the above kind in the animal kingdom, we should have to suppose that an animal whilst lying down determined to rise up in some par- ticular direction ; and that after its head had been cut off, an impulse continued to travel very slowly along the nerves to the proper muscles ; so that after several hours the headless animal rose up in the predeter- mined direction.

part which is sensitive to geotropism in the members of such distinct families as the Leguminose, Malvaces, Cucurbitacesee and Graminee, we may infer that this character is common to the roots of most seedling plants. Whilst a root is penetrating the ground, the tip must travel first ; and we can see the advantage of its being sensitive to geotropism, as it has to deter- mine the course of the whole root. Whenever the tip is deflected by any subterranean obstacle, it will also be an advantage that a considerable length of the root should be able to bend, more especially as the tip itself grows slowly and bends but little, so that the proper downward course may be soon recovered. But it appears at first sight immaterial whether this were effected by the whole growing part being sensitive to geotropism, or by an influence transmitted exclusively from the tip. We should, however, remember that it is the tip which is sensitive to the contact of hard objects, causing the radicle to bend away from them, thus guiding it along the lines of least resistance in the soil. It is again the tip which is alone sensitive, at least in some cases, to moisture, causing the radicle to bend towards its source. These two kinds of sensitiveness conquer for a time the sensitiveness to geotropism, which, however, ultimately prevails. Therefore, the three kinds of sensitiveness must often come into antagonism ; first one prevailing, and then another; and it would be an advantage, perhaps a necessity, for the interweighing and reconciling of these three kinds of sensitiveness, that they should be all localised in the same group of cells which have to transmit the command to the adjoining parts of the radicle, causing it to bend to or from the source of irritation.

the attraction of gravity has an important bearing on the theory of geotropism. Authors seem generally to look at the bending of a radicle towards the centre of the earth, as the direct result of gravitation, which is believed to modify the growth of the upper or lower surfaces, in such a manner as to induce curvature in the proper direction. But we now know that it is the tip alone which is acted on, and that this part trans- mits some influence to the adjoining parts, causing them to curve downwards. Gravity does not appear to act ina more direct manner on a radicle, than it does on any lowly organised animal, which moves away when it feels some weight or pressure.

Nature of the circumnutating movex.ent—History of a germinating seed — The radicle first protrudes and circumnutates—Its tip highly sensilive—Emergence of the hypocotyl or of the epicuty] from the ground under the form of an arch—Its circumnutation and that of the cotyledons—The seedling throws up a leaf-bearing stem—The circumnutation of all the parts or organs—Modified circumnutation—Epinasty and hyponasty—Movements of climbing plants —Nyctitropic movements—Movements excited by light and gravitation — Localised sensitiveness — Resemblance between the movements of plants and animals—The tip of the radicle acts like a brain.

Ir may be useful to the reader if we briefly sum up the chief conclusions, which, as far as we can judge, have been fairly well established by the observations given in this volume. All the parts or organs in every plant whilst they continue to grow, and some parts which are provided with pulvini after they have ceased to grow, are continually circumnutating. This movement commences even before the young seedling has broken through the ground. The nature of the movement and its causes, as far as ascertained, have been briefly described in the Introduction. Why every part of a plant whilst it is growing, and in some cases after growth has ceased, should have its cells rendered more turgescent and its cell-walls more extensile first on one side and then on another, thus inducing circumnutation, is not known. It would appear as if the changes in the cells required periods of rest.

In some cases, as with the hypocotyls of Brassica, the leaves of Dionza and the joints of the Graminea, the circumnutating movement when viewed under the microscope is seen to consist of innumerable small oscillations. ‘The part under observation suddenly jerks forwards for a length of -002 to -001 of an inch, and then slowly retreats for a part of this distance; after a few seconds it again jerks forwards, but with many intermissions. The retreating movement appa- rently is due to the elasticity of the resisting tissues. How far this oscillatory movement is general we do not know, as not many circumnutating plants were observed by us under the microscope; but no such movement could be detected in the case of Drosera with a 2-inch object-glass which we used. The pheno- menon is a remarkable one. The whole hypocotyl of a cabbage or the whole leaf of a Dionwa could not jerk forwards unless a very large number of cells on one side were simultaneously affected. Are we to sup- pose that these cells steadily become more and more turgescent on one side, until the part suddenly yields and bends, inducing what may be called a micro- scopically minute earthquake in the plant; or do the cells on one side suddenly become turgescent in an intermittent manner; each forward movement thus caused being opposed by the elasticity of the tissues ?

Circumnutation is of paramount importance in the life of every plant; for it is through its modification that many highly beneficial or necessary movements have been acquired. When light strikes one side of a plant, or light changes into darkness, or when gravitation acts on a displaced part, the plant is enabled in some unknown manner to increase the always varying turgescence of the cells on one side; so that the ordinary circumnutating movement is

modified, and the part bends either to or from the exciting cause; or it may occupy a new position, as in the so-called sleep of leaves. The influence which modifies circumnutation may be transmitted from one part to another. Innate or constitutional changes, independently of any external agency, often modify the circumnutating movements at particular periods of the life of the plant. As circumnutation is uni- versally present, we can understand how it is that movements of the same kind have been developed in the most distinct members of the vegetable series. But it must not be supposed that all the movements of plants arise from modified circumnutation ; for, as we shall presently see, there is reason to believe that this is not the case.

Having made these few preliminary remarks, we will in imagination take a germinating seed, and con- sider the part which the various movements play in the life-history of the plant. The first change is the protrusion of the radicle, which begins at once to circumnutate. This movement is immediately modi- fied by the attraction of gravity and rendered geo- tropic. The radicle, therefore, supposing the seed to be lying on the surface, quickly bends downwards, fol- lowing a more or less spiral course, as was seen on the smoked glass-plates. Scnsitiveness to gravitation re- sides in the tip; and it is the tip which transmits some influence to the adjoining parts, causing them to bend. As soon as the tip, protected by the root- vap, reaches the ground, it penetrates the surface, if this be soft or friable; and the act of penetration is apparently aided by the rocking or circumnutating movement of the whole end of the radicle. If the sur- face is compact, and cannot easily be penetrated, then