OBJECTIONS TO THE THEORY 833 



added; namely, sensitiveness to a touch, by which means 

 the foot-stalks of the leaves or flowers, or these modified 

 and converted into tendrils, are excited to bend round 

 and clasp the touching object. He who will read my 

 memoir on these plants will, I think, admit that all the 

 many gradations in function and structure between simple 

 twiners and tendril-bearers are in each case beneficial in 

 a high degree to the species. For instance, it is clearly 

 a great advantage to a twining -^lanX, to become a leaf- 

 climber; and it is probable that every twiner which 

 possessed leaves with long foot-stalks would have been 

 developed into a leaf-climber, if the foot-stalks had pos- 

 sessed in any slight degree the requisite sensitiveness to 

 a touch. 



As twining is the simplest means of ascending a 

 support, and forms the basis of our series, it may natu- 

 rally be asked how did plants acquire this power in an 

 incipient degree, afterward to be improved and increased 

 through natural selection? The power of twining depends, 

 5rst, on the stems while young being extremely flexible 

 but this is a character common to many plants which are 

 lot climbers); and, secondly, on their continually bending 

 o all points of the compass, one after the other in suc- 

 ession, in the same order. By this movement the stems 

 re inclined to all sides, and are made to move round 

 nd round. As soon as the lower part of a stem strikes 

 gainst any object and is stopped, the upper part still 

 oes on bending and revolving, and thus necessarily 

 ines round and up the support. The revolving move- 

 ent ceases after the early growth of each shoot. As in 

 any widely separated families of plants, single species 

 [id single genera possess the power of revolving, and 



