306 MISCELLANEOUS OBJECTIONS TO THE [Chap. VIL 



two latter classes the steins have generally, but not 

 always, lost the power of twining, though they retain the 

 power of revolving, which the tendrils likewise possess. 

 The gradations from leaf-climbers to tendril-bearers are 

 wonderfully close, and certain plants may be indifferent- 

 ly placed in either class. But in ascending the series 

 from simple twiners to leaf -climbers, an important qual- 

 ity is 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 bene- 

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

 clearly a great advantage to a twining plant 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 

 possessed in any slight degree the requisite sensitiveness 

 to a touch. 



As twining is the simplest means of ascending a sup- 

 port, and forms the basis of our series, it may naturally 

 be asked how did plants acquire this power in an in- 

 cipient degree, afterwards to be improved and increased 

 through natural selection. The power of twining de- 

 pends, firstly, on the stems whilst young being extremely 

 flexible (but this is a character common to many plants 

 which are not climbers); and, secondly, on their con- 

 tinually bending to all 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 



