THEORY OF NATURAL SELECTION. 219 



to the plants in question ; anyhow, they are not of the least 

 use in the way of climbing, which is the point that concerns 

 us. Nevertheless we can see that if the stems of these 

 plants had been flexible, and if under the conditions to 

 which they are exposed it had profited them to ascend to 

 a height, then the habit of slightly and irregularly revolv- 

 ing might have been increased and utilized through natural 

 selection, until they had become converted into well-developed 

 twining species. 



With respect to the sensitiveness of the foot-stalks of the 

 leaves and flowers, and of tendrils, nearly the same remarks 

 are applicable as in the case of the revolving movements of 

 twining plants. As a vast number of species, belonging to 

 widely distinct groups, are endowed with this kind of sen- 

 sitiveness, it ought to be found in a nascent condition in 

 many plants which have not become climbers. This is the 

 case. I observed that the young flower-peduncles of the 

 above Maurandia curved themselves a little toward 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, toward 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 

 sensitive to these stimuli. So with plants it appears that, 

 from having the power of movement in obedience to certain 



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