The Power of Movement in Plants. Charles Darwin
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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 Jacobaeus the cells at first increase a little in length; in Oxalis corniculata they decrease a little, owing to self-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 [page 123] 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 subsequent contraction of their walls, we can perhaps understand 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 dependent on their action may be long-continued without 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 Light.—The hypocotyls and cotyledons of most seedling 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. [page 124]
[For instance, the cotyledons of Beta vulgaris, Solanum lycopersicum, Cerinthe major, and Lupinus luteus, when placed in darkness, moved down during the afternoon and early night, instead of rising as they would have done if they had been exposed to 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 Oxalis 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 Jacoboeus were not affected by 4 h. 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 40 h. Other seedlings were placed in the dark after their cotyledons had opened in the morning and these did not begin to close until about 4 h. had elapsed. The cotyledons of Oxalis rosea sank vertically downwards after being left for 1 h. 20 m. 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 1 h. 13 m. they began to open. 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 2 h. to dull sunshine, and the cotyledons [page 125] remained horizontal; it was then brought back to the table, and after 50 m. the cotyledons had risen 68o above the horizon. The other pot was placed during the same 2 h. behind a screen in the room, where the light was very obscure, and the cotyledons rose 63o above the horizon; the pot was then replaced on the table, and after 50 m. the cotyledons had fallen 33o. These two pots with seedlings of the same age stood close together, and were exposed to exactly the same amount of light, yet the cotyledons 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 seedlings, 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 darkness; yet the cotyledons were partially open in the morning and remained open in the dark for about 6 h. The cotyledons in another pot, similarly treated on another occasion, were open at 7 A.m. and remained open in the dark for 4 h. 30 m., 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 cotyledons of many others. The genus cassia seems to be pre-eminent in this respect: thus, the cotyledons of C. tora, when extended horizontally, were both lightly tapped with a very thin twig for 3 m. and in the course of a few minutes they formed together an angle of 90o, so that each had risen 45o. A single cotyledon of another seedling was tapped in a like manner for 1 m., and it rose 27o in 9 m.; and after eight additional minutes it had risen 10o more; the opposite cotyledon, which was not tapped, hardly moved at all. The cotyledons in all these cases became horizontal again in less than half an hour. The pulvinus is the most sensitive part, for on slightly pricking three cotyledons with a [page 126] 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 cotyledons 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 S. Brazil) rose 31o in the course of 26 m. after the pulvini and the blades had both been rubbed during 1 m. with a twig; but when the blade alone was similarly rubbed the cotyledons rose only 8o. The remarkably long and narrow cotyledons, of a third unnamed species from S. Brazil, did not move when their blades were rubbed on six occasions with a pointed stick for 30 s. or for 1 m.; 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 60o. Several cotyledons of C. neglecta (likewise from S. Brazil) rose in from 5 m. to 15 m. to various angles between 16o and 34o, after being rubbed during 1 m. 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. 50o 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. glauca were either rubbed with a thin twig for 2 m. or were lightly pinched: one rose 34o; a second only 6o; a third 13o; and a fourth 17o. A cotyledon of C. florida similarly