The Power of Movement in Plants. Charles Darwin

sensitiva.—This plant belongs to a distinct sub-order of the Leguminosae from Cassia. Both cotyledons of an oldish seedling, with the first true leaf partially unfolded, were rubbed for 1 m. with a fine twig, and in 5 m. each rose 32o; they [page 127] remained in this position for 15 m., but when looked at again 40 m. after the rubbing, each had fallen 14o. Both cotyledons of another and younger seedling were lightly rubbed in the same manner for 1 m., and after an interval of 32 m. each had risen 30o. They were hardly at all sensitive to a fine jet of water. The cotyledons of S. Pfundii, 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 1 m. or 2 m.; but they did not move in the least. When, however, the pulvini of six cotyledons of M. 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 rose a little. It thus appears that the cotyledons of Mimosa are less sensitive than those of the previously mentioned plants.*

      Oxalis sensitiva.—The blades and pulvini of two cotyledons, standing horizontally, were rubbed or rather tickled for 30 s. with a fine split bristle, and in 10 m. each had risen 48o; when looked at again in 35 m. after being rubbed they had risen 4o more; after 30 additional minutes they were again horizontal. On hitting a pot rapidly with a stick for 1 m., the cotyledons of two seedlings were considerably raised in the course of 11 m. 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 56o, a second 45o, a third almost 90o, and a fourth 90o. After an additional interval of 40 m. three of them had re-expanded to a considerable extent. These observations were made before we were aware at what an extraordinarily 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.]

      Finally, there seems to exist some relation between

      * The sole notice which we have met with on the sensitiveness of cotyledons, relates to Mimosa; for Aug. P. De Candolle says ('Phys. Vég.,' 1832, tom. ii. p. 865), "les cotyledons du M. pudica tendent à se raprocher par leurs faces supérieures lorsqu'on les irrite." [page 128]

      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 connected; but this is not necessarily the case, for the cotyledons of Oxalis sensitiva did not rise when kept on one occasion for 1½ h., and on a second occasion for nearly 4 h., in a dark closet. Some other cotyledons, 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 described 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 dependent position. But the relation between the sensitiveness 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. [page 129]