232 MISCELLANEOUS OBJECTIONS TO THE 



We will now turn to climbing plants. These can be ar- 

 ranged in a long series, from those which simply twine 

 round a support, to those which I have called leaf-climbers, 

 and to those provided with tendrils. In these two latter 

 classes the stems have generally, but not always, lost the 

 power of twining, though they retain the power of revolv- 

 ing, which the tendrils likewise possess. The gradations 

 from leaf-climbers to tendril bearers are wonderfully close, 

 and certain plants may be differently placed in either 

 class. But in ascending the series from simple twiners to 

 leaf-climbers, an important quality is added, namely sen- 

 sitiveness to a touch, by which means the foot-stalks of the 

 leaves or flowers, or these modified and converted into ten- 

 drils, 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 possessed in any slight degree the requisite sen- 

 sitiveness to a touch. 



As twining is the simplest means of ascending a support, 

 and forms the basis of our series, it may naturally 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, firstly, on the 

 stems while young being extremely flexible (but this is a 

 character common to many plants which are not climbers) ; 

 and, secondly, on their continually 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 part of a stem strikes against any object and 

 is stopped, the upper part still goes on bending and revolv- 

 ing, 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 families of 

 plants, single species and single genera possess the power 

 qf revolving, and have thus become twiners, they must have 



