The Power of Movement in Plants

above it. After 24 days from the first observation (begun after a true leaf had been developed) the cotyledons ceased to rise at night. i alis (Biophytum) sensitiva.—The cotyledons of several seed- lings, 45 days after their first expansion, stood nearly vertical at night, and closely embraced either one or two true leaves which by this time had been formed. These seedlings had been kept in a very warm house, and their development had been rapid.

Oxalis corniculuta.—The cotyledons do not stand vertical at night, but generally rise to an angle of about 45° above the horizon. They continued thus to act for 23 days after their first expansion, by which time two leaves had been formed ; even after 29 days they still rose moderately above their hori- zontal or downwardly deflected diurnal position. Mimosa pudica.—The cotyledons were expanded for the first time on Nov. 2nd, and stood vertical at night. On the 15th the first leaf was formed, and at night the cotyledons were vertical. On the 28th they behaved in the same manner. On Dec. 15th, that is after 44 days, the cotyledons were still considerably raised at night; but those of another seedling, only one day older, were raised very little.

Mimosa albida.—A seedling was observed during only 12 days, by which time a leaf had been formed, and the cotyledons were then quite vertical at night. Trifolium subterraneum.—A seedling, 8 days old, had its coty- ledons horizontal at 10.30 a.m.and vertical at 9.15 p.m. After an interval of two months, by which time the first and second true leaves had been developed, the cotyledons still performed the same movement. They had now increased greatly in size, and had become oval; and their petioles were actually ‘8 of an inch in length!

Trifolium strictum.—After 17 days the cotyledons still rose at night, but were not afterwards observed. Lotus Jacobeeus—The cotyledons of some seedlings having well-developed leaves rose to an angle of about 45° at night; and even after 3 or 4 whorls of leaves had been formed, the co- tyledons rose at night considerably above their diurnal hori- zontal position. Cassia mimosoides—The cotyledons of this Indian spccics, 14 days after their first expansion, and when a leaf had becn formed, stood during the day horizontal, and at night vertical.

Cassia sp ? (a large S. Brazilian tree raised from seeds sent us by F. Miiller).—The cotyledons, after 16 days from their first expansion, had increased greatly in size with two leaves just formed: They stood horizontally during the day and vertically at night, but were not afterwards observed. Cassia neglecta (likewise a 8, Brazilian species)—A seedling, 34 days after the first expansion of its cotyledons, was between 3 and 4 inches in height, with 3 well-developed leaves; and the cotyledons, which during the day were nearly horizontal, at night stood vertical, closely embracing the young stem. The cotyle- dons of another seedling of the same age, 5 inches in height, with 4 well-developed leaves, behaved at night in exactly the same manner.

It is known * that there is no difference in structure between the upper and lower halves of the pulvini of leaves, sufficient to account for their upward or down- ward movements. In this respect cotyledons offer an unusally good opportunity for comparing the structure of the two halves; for the cotyledons of Oxalis Valdi- viana rise vertically at night, whilst those of O. rosea sink vertically ; yet when sections of their pulvini were made, no clear difference could be detected between the corresponding halves of this organ in the two species which move so differently. With 0. rosea, however, there were rather more cells in the lower than in the upper half, but this was likewise the case in one speci- men of O. Valdiviana. The cotyledons of both species (83 mm. in length) were examined in the morning whilst extended horizontally, and the upper surface of the pulvinus of O. rosea was then wrinkled transversely, showing that it was in a state of compression, and this might have been expected as the cotyledons sink at

night with O. Valdiviana it was the lower surface which was wrinkled, and its cotyledons rise at night. the species seen by us are pulvinated; so it is with the cotyledons of T. subterraneum and strictum, which stand vertically at night; whereas those of 7. reswpi- natum exhibit not a trace of a pulvinus, nor of any nocturnal movement. This was ascertained by mea- suring the distance between the tips of the cotyledons of four seedlings at mid-day and at night. In this species, however, as in the others, the first-formed leaf, which is simple or not trifoliate, rises up and sleeps like the terminal leaflet on a mature plant.

In another natural genus, Oxalis, the cotyledons of 0. Valdiviana, rosea, floribunda, articulata, and sensitiva are pulvinated, and all move at night into an upward or downward vertical position. In these several species the pulvinus is seated close to the blade of the coty- ledon, as is the usual rule with most plants. Ozalis cor- niculata (var. Atro-purpurea) differs in several respects ; the cotyledons rise at night to a very variable amount, rarely more than 45°; and in one lot of seedlings (purchased under the name of O. tropeoloides, but certainly belonging to the above variety) they rose only from 5° to 15° above the horizon. The pulvinus is developed imperfectly and to an extremely variable degree, so that apparently it is tending towards abor- tion. No such case has hitherto, we believe, been described. It is coloured green from its cells con- taining chlorophyll; and it is’ seated nearly in the middle of the petiole, instead of at the upper end as in all the other species. The nocturnal movement is effected partly by its aid, and partly by the growth of the upper part of the petiole as in the case of plants destitute of a pulvinus. Fr-m these several reasons and from our having partially traced the develop- ment of the pulvinus from an early age, the case seems worth describing in some detail.

When the cotyledons of 0. cornicudutx were dissected out of a seed from which they would soon have naturally emerged, no trace of a pulvinus could be detected ; and all the cells forming the short petiole, 7 in number in a longitudinal row, were of nearly equal size. In seedlings one or two days old, the pulvinus was so indistinct that we thought at first that it did not exist; but in the middle of the petiole an ill-defined transverse zone of cells could be seen, which were much shorter than those both above and below, although of the same breadth with them. They presented the appearance of having been just formed by the transverse division of longer cells; and there can be little doubt that this had occurred, for the cells in the petiole which had

Oxalis corniculata: A and B the almost rudimentary pulvini of the coty- leduons of two rather old seedlings, viewed as transparent objects. Magnified 50 times. been dissected out of the seed averaged in length 7 divisions of the micrometer (each division equalling ‘003 mm ), and were a little longer than those forming a well-developed pulvinus, which varied between 4 and 6 of these same divisions. After a faw additional days the ill-defined zone of cells becomes distinct, and although it does not extend across the whole width of the petiole, and although the cells are of a green colour from contain- ing chlorophyll, yet they certainly constitute a pulvinus, which, as we shall presently see, acts as one. These small cells were arranged in longitudinal rows, and varied from 4 to7 in number; and the cells themselves varied in length in different ; arts of the

same pulvinus and in different individuals. In the accompany- ing figures, A and B (Fig. 64), we have views of the epidermis ‘3 in the middle part of the petioles of two seedlings, in which the pulvinus was for this species well developed. They offer a striking contrast with the pulvinus of O. rosea (see former Fig. 63), or of O. Valdiviana, With the seedlings, falsely called O. tropeoloides, the cotyledons of which rise very little at night, the small cells were still fewer in number and in parts formed a single transverse row, and in other parts short longitudinal rows of only two or three. Nevertheless they sufficed to attract the eye, when the whole petiole was viewed as a transparent object beneath the microscope. In these seedlings there could hardly be a doubt that the pulvinus was becoming rudimentary and tending to disappear; and this accounts for its great variability in structure and function.

-In the following Table some measurements of the cells in fairly well-developed pulvini of 0. corniculata are given :— Average length of cells of pulvinus Sar ola eee war OORT Length of longest cell below the pulvinus .. .. .. 13 Length of longest cell above the pulvinus .. .. .. 20 Seedling 5 dys old, cotyledon 3:1 mm. in length, with the puloinus quite distinct. Average lencth of cells of pulvinus sa ES Cie 6 Length of longest cell below the pulvinus .. .. .. 22 Leng«h of longest cell above the pulvinus .. .. 40

Seedling 8 days 0’d, cotyledon 5 mm. in length, with a true leaf formed but not yet expanded, Seedling 13 days old, cotyledon 4:5 mm. in lengti, witha small true leaf fully developed. Average length of cells of pulvinus ae. Gb) dee 7 Length of longest cell below the pulvinus .. .. .. 30 Length of longest cell above the pulvinus .. .. .. co * Longiludinal sections show pulvinus. that the forms of the epidermic + Each division cqualled +003 culls may be taken as a fairrepre- mm, sentation of those constituting the °

We here see that the cells of the pulvinus increase but little in length with advancing age, in comparison with those of the petiole both above and below it; but they continue to grow in width, and keep equal in this respect with the other cells of the petiole. The rate of growth, however, varies in all parts of the cotyledons, as may be observed in the measurements of the 8-days’ old seedling. The cotyledons of seedlings only a day old rise at night con- siderably, sometimes as much as afterwards; but there was much variation in this respect. As the pulvinus is so indistinct at first, the movement probably does not then depend on the expansion of its cells, but on periodically unequal growth in the petiole. By the comparison of seedlings of different known ages, it was evident that the chief seat of growth of the petiole was in the upper part between the pulvinus and the blade; and this agrees with the fact (shown in the measurements above given) that the cells grow to a greater length in the upper than in the lower part. With a seedling 11 days old, the nocturnal rise was found to depend largely on the action of the pulvinus, for the petiole at night was curved upwards at this point; and during the day, whilst the petiole was horizontal, the lower surface of the pulvinus was wrinkled with the upper surface tense. Although the cotyledons at an advanced age do not rise at night toa higher inclination than whilst young, yet they have to pass through a larger angle (in one instance amounting to 63°) to gain their nocturnal position, as they are generally deflected beneath the horizon during the day. Even with the 1l-days’ old seedling the movement did not depend exclusively on the pulvinus, for the blade where joined to the petiole was curved upwards, and this must be attributed to unequal growth, Therefore the periodic movements of the cotyledons of O. corni- culala depend on two distinct but conjoint actions, namely, the expansion of the cells of the pulvinus and on the growth of the upper part of the petiole, including the base of the blade.

Lotus Jacobeus.—The seedlings of this plant present a case parallel to that of Oxalis corniculata in some respects, and in others unique, as far as we have seen. The cotyledons during the first 4 or 5 days of their life do not exhibit any plain noc- turnal movement; but afterwards they stand vertically or almost vertically up at night. There is, however, some degree of variability in this respect, apparently dependent on the season and on the degree to which they have been illuminated during

the day. With older secdlings, having cotyledons 4 mm, in length, which rise considerably at night, there is a well-deve- loped pulvinus close to the blade, colourless, and rather nar- rower than the rest of the petiole, from which it is abruptly separated. It is formed of a mass of small cells of an average length of 021 mm.; whereas the cells in the lower part of the petiole are about ‘06 mm., and those in the blade from -034 to ‘04mm. in length. The epidermic cells in the lower part of the petiole project conically, and thus differ in shape from those over the pulvinus.

Turning now to very young seedlings, the cotyledons of which do not rise at night and are only from 2 to 24 mm. in length, their petioles do not exhibit any defined zone of small cells, destitute of chlorophyll and differing in shape exteriorly from the lower ones. Nevertheless, the cells at the place where a pulvinus will afterwards be developed are smaller (being on an average ‘015 mm. in length) than those in the lower parts of the same petiole, which gradually become larger in proceeding downwards, the largest being ‘0830 mm. in length. At this early age the cells of the blade are about ‘027 mm. in length. We thus see that the pulvinus is formed by the cells in the upper- most part of the petiole, continuing for only a short time to increase in length, then being arrested in their growth, accom- panied by the loss of their chlorophyll grains; whilst the cells in the lower part of the petiole continue for a long time to increase in length, those of the epidermis becoming more conical. The singular fact of the cotyledons of this plant not sleeping at first is therefore due to the pulvinus not being developed at an early age.

We learn from these two cases of Lotus and Oxalis, that the development of a pulvinus follows from the growth of the cells over a small defined space of the petiole being almost arrested at an early age. With Lotus Jacobeus the cells at first increase a little in length; in Ozalis corniculata they decrease a. little, owing to seli-division. A mass of such small cells forming a pulvinus, might therefore be either acquired or lost without any special difficulty, by different species in the same natural genus: and we know that

with seedlings of Trifolium, Lotus, and Oxalis some of the species have a well-developed pulvinus, and others have none, or one in a rudimentary condition. As the movements caused by the alternate turgescence of the cells in the two halves of a pulvinus, must be largely determined by the extensibility and subse- quent contraction of their walls, we can perhaps under- stand why a large number of small cells will be more efficient than a small number of large cells occupying the same space. As a pulvinus is formed by the arrestment of the growth of its cells, movements de- pendent on their action may be long-continued withou any increase in length of the part thus provided; and such long-continued movements seem to be one chief end gained by the development of a pulvinus. Long-continued movement would be impossible in any part, without an inordinate increase in its length, if the turgescence of the cells was always followed by growth.

Disturbance of the Periodic Movements of Cotyledons by Inght.—The hypocotyls and cotyledons of most seed- ling plants are, as is well known, extremely heliotropic ; but cotyledons, besides being heliotropic, are affected paratonically (to use Sachs’ expression) by light; that is, their daily periodic movements are greatly and quickly disturbed by changes in its intensity or by its absence. It is not that they cease to circumnutate in darkness, for in all the many cases observed by us they continued to do so; but the normal order of their movements in relation to the alternations of day and night is much disturbed or quite annulled. This holds good with species the cotyledons of which rise or sink so much at night that they may be said to sleep, as well as with others which rise only a little. But different species are affected in very different degrees by changes in the light.

For instance, the cotyledons of Beta vulyaris, Solanum lycoper- sicum, Cerinthe major, and Lupinus luteus, when placed in dark- ness, moved down during the afternoon and early night, instead of rising as they would have done if they had been exposed tc the light. All the individuals of the Solanum did not behave in the same manner, for the cotyledons of one circumnutated about the same spot between 2.30 and 10 p.m. The cotyledons of a seedling of Uxalis corniculata, which was feebly illuminated from above, moved downwards during the first morning in the normal manner, but on the second morning it moved upwards. The cotyledons of Lotus Jacobeus were not affected by 4h. of complete darkness, but when placed under a double skylight and thus feebly illuminated, they quite lost their periodical movements on the third morning. On the other hand, the cotyledons of Cucurbita ovifera moved in the normal manner during a whole day in darkness.

Seedlings of Githago segetum were feebly illuminated from above in the morning before their cotyledons had expanded, and they remained closed for the next 40h. Other seedlings were placed in the dark after their cotyledons had opened in the morning and these did not begin to close until about 4h. had elapsed. The cotyledons of Oxulis rosca sank vertically down- wards after being left for 1h. 20m. in darkness; but those of some other species of Oxalis were not affected by several hours of darkness. The cotyledons of several species of Cassia are eminently susceptible to changes in the degree of light to which they are exposed: thus seedlings of an unnamed S. Brazilian species (a large and beautiful tree) were brought out of the hot- house and placed on a table in the middle of a room with two north-east and one north-west window, so that they were fairly well illuminated, though of course less so than in the hot-house, the day being moderately bright; and after 36 m. the cotyledons which had been horizontal rose up vertically and closed together as when asleep; after thus remaining on the table for 1h. 13 m. they began toopen. The cotyledons of young seedlings of another Brazilian species and of C. neglecta, treated in the same manner, behaved similarly, excepting that they did not rise up quite so much; they again became horizontal after about an hour.

Here is a more interesting case: seedlings of Cassia tora in two pots, which had stood for some time on the table in the room just described, had their cotyledons horizontal. One pot was now exposed for 2h. to dull sunshine, and the cotyledons remained horizontal; it was then brought back to the table, and after 50m. the cotyledons had risen 68° above the horizon. The other pot was placed during the same 2h. behind a screen in the room, where the light was very obscure, and the cotyledons rose 63° above the horizon; the pot was then replaced on the table, and after 50 m. the cotyledons had fallen 83°. These two pots with seedlings of the same age stood close together, and were exposed to exactly the same amount of light, yet the coty- ledons in the one pot were rising, whilst those in the other pot were at the same time sinking. This fact illustrates in a striking manner that their movements are not governed by the actual amount, but by a change in the intensity or degree of the light. A similar experiment was tried with two sets of seed- lings, both exposed to a dull light, but different in degree, and the result was the same. The movements of the cotyledons of this Cassia are, however, determined (as in many other cases) largely by habit or inheritance, independently of light; for seedlings which had been moderately illuminated during the day, were kept all night and on the following morning in complete dark- ness; yet the cotyledons were partially open in the morning and remained open in the dark for about 6h. The cotyledons in another pot, similarly treated on another occasion, were open at 7 a.m. and remained open in the dark for 4h. 30m, after which time they began to close. Yet these same seedlings, when brought in the middle of the day from a moderately bright into only a moderately dull light raised, as we have seen, their cotyledons high above the horizon.

Sensitiveness of Cotyledons to contact.—This subject does not possess much interest, as it is not known that sensitiveness of this kind is of any service to seedling plants. We have observed cases in only four genera, though we have vainly observed the coty- ledons of many others. The genus Cassia seems to be pre-eminent in this respect: thus, the cotyledons of OC. tora, when extended horizontally, were both lightly tapped with a very thin twig for 3m., and in the course of a few minutes they formed together an angle of 90°, so that each had risen 45°. A single cotyledon of another seedling was tapped in a like manner for 1 m., and it rose 27° in 9m.; and after eight additional minutes it had risen 10° more; the opposite cotyledon, which was not tapped, hardly moved at all, The cotyledons in all these cases became hori- zontal again in less than half an hour. The pulvinus is the most sensitive part, for on slightly pricking three cotyledons with e

pin in this part, they rose up vertically ; but the blade was found also to be sensitive, care having been taken that the pulvinus was not touched. Drops of water placed quietly on these coty- ledons produced no effect, but an extremely fine stream of water, ejected from a syringe, caused them to move upwards. When a pot of seedlings was rapidly hit with a stick and thus jarred, the cotyledons rose slightly. When a minute drop of nitric acid was placed on both pulvini of a seedling, the cotyledons rose so quickly that they could easily be seen to move, and almost immediately afterwards they began to fall; but the pulvini had been killed and became brown.

The cotyledons of an unnamed species of Cassia (a large tree from 8. Brazil) rose 31° in the course of 26 m. after the pulvini and the blades had both been rubbed during 1m. with a twig; but when the blade alone was similarly rubbed the cotyledons rose only 8°. The remarkably long and narrow cotyledons, of a third unnamed species from 8. Brazil, did not move when their blades were rubbed on six occasions with a pointed stick for 30s. or for 1m.; but when the pulvinus was rubbed and slightly pricked with a pin, the cotyledons rose in the course of a few minutes through an angle of 60°. Several cotyledons of C. neglecta (likewise from 8S. Brazil) rose in from 5 m. to 15 m. to various angles between 16° and 84°, after being rubbed during 1m. with a twig. Their sensitiveness is retained to a somewhat advanced age, for the cotyledons of a little plant of C. neglecta, 34 days old and bearing three true leaves, rose when lightly pinched between the finger and thumb. Some seedlings were exposed for 30 m. to a wind (temp. 50° F.) sufficiently strong to keep the cotyledons vibrating, but this to our surprise did not cause any movement. The cotyledons of four seedlings of the Indian C. gluuca were either rubbed with a thin twig for 2m. or were lightly pinched: one rose 34°; a second only 6°; a third 13°; and a fourth 17°. A cotyledon of C. floridu similarly treated rose 9°; one of C. corymhosa rose 73°, and one of the very distinct C. mimosoides only 6°. Those of C. pubescens did not appear to be in the least sensitive; nor were those of C. ncdosa, but these latter are rather thick and fleshy, and do not rise at night or go to sleep.

Smithia sensitiva.—This plant belongs to a distinct sub-order of the Leguminose from Cassia. Both cotyledons of an oldish seedling, with the first true leaf partially unfolded, were rubbed for 1m. with a fine twig, and in 5m. each rose 32°; thoy remained in this position for 15m., but when looked at again 40m. after the rubbing, each had fallen 14°. Both cotyledons of another and younger seedling were lightly rubbed in the same manner for 1m., and after an interval of 82 m. each had risen 30°. They were hardly at all sensitive to a fine jet of water. The cotyledons of S. 2?fundii, an African water plant, are thick and fleshy ; they are not sensitive and do not go to sleep.

Mimosa pudica and albida—The blades of several cotyledons of both these plants were rubbed or slightly scratched with a needle during 1m. or 2m.; but they did not move in the least. When, however, the pulvini of six cotyledons of MM. pudica were thus scratched, two of them were slightly raised. In these two cases perhaps the pulvinus was accidentally pricked, for on pricking the pulvinus of another cotyledon it rosea little. It thus appears that the cotyledons of Mimosa are less sensitive than those of the previously mentioned plants.*

Ozalis sensitivuk—The blades and pulvini of two cotyledons, standing horizontally, were rubbed or rather tickled for 30s, with a fine split bristle, and in 10m. each had risen 48°; when looked at again in 385 m. after being rubbed they had risen 4° more; after 30 additional minutes they were again hori- zontal, On hitting a pot rapidly with a stick for 1m., the coty- ledons of two seedlings were considerably raised in the course of llm. A pot was carried a little distance on a tray and thus jolted; and the cotyledons of four seedlings were all raised in 10 m.; after 17 m. one had risen 56°, a second 45°, a third almost 90°, and a fourth 90°. After an additional interval of 40 m. three of them had re-expanded to a considerable extent. These obser- vations were made before we were aware at what an extraordi- narily rapid rate the cotyledons circumnutate, and are therefore liable to error. Nevertheless it is extremely improbable that the cotyledons in the eight cases given, should all have been rising at the time when they were irritated. The cotyledons of Oxalis Valdiviana and rosea were rubbed and did not exhibit any sensitiveness,

* The sole notice which we p. 865), “les cotyledons du M have met with on tle sensitive- pudica tendent a se raprocher pay ness of cotyledons, relates to Mi- leurs faces supérieures lorsqu’on mosa; for Aug. P. De Candolle les irrite.” vays (‘ Phys. Vég.,’ 1832, tom. ii. the habit of cotyledons rising vertically at night or going to sleep, and their sensitiveness, especially that of their pulvini, to a touch ; for all the above-named plants sleep at night. On the other hand, there are many plants the cotyledons of which sleep, and are not in the least sensitive. As the cotyledons of several species of Cassia are easily affected both by slightly diminished light and by contact, we thought that these two kinds of sensitiveness might be con- nected; but this is not necessarily the case, for the cotyledons of Ozalis sensitiva did not rise when kept on one occasion for 1§ h., and on a second occasion for nearly 4h., in a dark closet. Some other coty- ledons, as those of Githago segetum, are much affected by a feeble light, but do not move when scratched by a needle. That with the same plant there is some relation between the sensitiveness of its cotyledons and leaves seems highly probable, for the above de- scribed Smithia and Oxalis have been called sensitiva, owing to their leaves being sensitive; and though the leaves of the several species of Cassia are not sensitive to a touch, yet if a branch be shaken or syringed with water, they partially assume their nocturnal de- pendent position. But the relation between the sen- sitiveness to contact of the cotyledons and of the leaves of the same plant is not very close, as may be inferred from the cotyledons of Mimosa pudica being only slightly sensitive, whilst the leaves are well known to be so in the highest degree. Again, the leaves of Neptunia oleracea are very sensitive to a touch, whilst the cotyledons do not appear to be so in any degree.