200 THE ORIGIN OF SPECIES 



points of the compass, one after the other in succession, in the 

 same order. By this movement the stems are inclined to all sides, 

 and are made to move round and round. As soon as the lower part 

 of a stem strikes against any object and is stopped, the upper part 

 still goes on bending and revolving, and thus necessarily twines 

 round and up the support. The revolving movement ceases after 

 the early growth of each shoot. As in many widely separated fam- 

 ilies of plants, single species and single genera possess the power 

 of revolving, and have thus become twiners, they must have inde- 

 pendently acquired it, and cannot have inherited it from a com- 

 mon progenitor. Hence, I was led to predict that some slight tend- 

 ency to a movement of this kind would be found to be far from 

 uncommon with plants which did not climb; and that this had 

 afforded the basis for natural selection to work on and improve. 

 When I made this prediction, I knew of only one imperfect case, 

 namely, of the young flower-peduncles of a Maurandia which 

 revolved slightly and irregularly, like the stems of twining plants, 

 but without making any use of this habit. Soon afterward Fritz 

 Miiller discovered that the young stems of an Alisma and of a 

 Linum — plants which do not climb and are widely separated in 

 the natural system — revolved plainly, though irregularly: and he 

 states that he has reason to suspect that this occurs with some 

 other plants. These slight movements appear to be of no service 

 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. Neverthe- 

 less 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 revolving 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 sensitiveness, 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 



