306 MISCELLANEOUS OBJECTIONS TO THE [Chap. TIL 



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 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 pos- 

 sessed 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 incipient 

 degree, afterwards to be improved and increased through 

 natural selection. The power of twining depends, 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 continually bend- 

 ing to all points of the compass, one after the other in 

 succession, in the same order. Bv 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 move- 

 ment ceases after the earlv growth of each shoot. As 

 in many widely separated families 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 common progenitor. Hence I was led to predict that 



