Seite - 254 - in The Origin of Species

254 ORIGIN OF SPECIES plants which have not become climbers. This is the case: I observed that the young flower-peduncles of the above Mau- randia curved themselves a little towards the side which was touched. Morren found in several species of Oxalis that the leaves and their foot-stalks moved, especially after exposure to a hot sun, when they were gently and repeatedly touched, or when the plant was shaken. I repeated these observations on some other species of Oxalis with the same result; in some of them the movement was distinct, but was best seen in the young leaves ; in others it was extremely slight. It is a more important fact that according to the high authority of Hofmeister, the young shoots and leaves of all plants move after being shaken; and with climbing plants it is, as we know, only during the early stages of growth that the foot- stalks and tendrils are sensitive. It is scarcely possible that the above slight movements, due to a touch or shake, in the young and growing organs of plants, can be of any functional importance to them. But plants possess, in obedience to various stimuli, powers of movement, which are of manifest importance to them; for instance, towards and more rarely from the light,—in oppo- sition to, and more rarely in the direction of, the attraction of gravity. When the nerves and muscles of an animal are excited by galvanism or by the absorption of strychnine, the consequent movements may be called an incidental result, for the nerves and muscles have not been rendered specially sen- sitive to these stimuli. So with plants it appears that, from having the power of movement in obedience to certain stim- uli, they are excited in an incidental manner by a touch, or by being shaken. Hence there is no great difficulty in ad- mitting that in the case of leaf-climbers and tendril-bearers, it is this tendency which has been taken advantage of and in- creased through natural selection. It is, however, probable, from reasons which I have assigned in my memoir, that this will have occurred only with plants which had already ac- quired the power of revolving, and had thus become twiners. I have already endeavoured to explain how plants became twiners, namely, by the increase of a tendency to slight and irregular revolving movements, which were at first of no use to them; this movement, as well as that due to a touch or