194 CONCLUDING REMARKS. Chap. V. 



their terminal leaflets converted into perfect tendrils ; 

 in Eccremocarpus I have seen a single lateral branch 

 of a tendril replaced by a perfect leaflet ; in Vicia 

 sativa, on the other hand, leaflets are sometimes 

 replaced by tendril-branches ; and many other snch 

 cases could be given. But he who believes in the 

 slow modification of species will not be content simply 

 to ascertain the homological nature of different kinds 

 of tendrils ; he will wish to learn, as far as is possible, 

 by what actual steps leaves, flower-peduncles, &c, have 

 had their functions wholly changed, and have come to 

 serve merely as prehensile organs. 



In the whole group of leaf-climbers abundant 

 evidence has been given that an organ, still subserv- 

 ing the functions of a leaf, may become sensitive to a 

 touch, and thus grasp an adjoining object. With 

 several leaf-climbers the true leaves spontaneously 

 revolve ; and their petioles, after clasping a support 

 grow thicker and stronger. We thus see that leaves 

 may acquire all the leading and characteristic qualities 

 of tendrils, namely, sensitiveness, spontaneous move- 

 ment, and subsequently increased strength. If their 

 blades or laminae were to abort, they would form true 

 tendrils. And of this process of abortion we can follow 

 every step, until no trace of the original nature of 

 the tendril is left. In Mutisia clematis, the tendril, in 

 shape and colour, closely resembles the petiole of one 

 of the ordinary leaves, together with the midribs of the 

 leaflets, but vestiges of the laniinse are still occasionally 

 retained. In four genera of the Fumariaceae we can 



