82 LEAF-CLIMBERS. Chap. II 



stiff petioles of Clematis flammula are sensitive to even 

 much less weight # if spread over a wide surface. The 

 petioles always bend towards the side which is pressed 

 or touched, at different rates in different species, 

 sometimes within a few minutes, but generally after 

 a much longer period. After temporary contact with 

 any object, the petiole continues to bend for a con- 

 siderable time ; afterwards it slowly becomes straight 

 again, and can then re-act. A petiole excited by an 

 extremely slight weight sometimes bends a little, and 

 then becomes accustomed to the stimulus, and either 

 bends no more or becomes straight again, the weight 

 still remaining suspended. Petioles which have clasped 

 an object for some little time cannot recover their 

 original position. After remaining clasped for two or 

 three days, they generally increase much in thickness 

 either throughout their whole diameter or on one side 

 alone ; they subsequently become stronger and more 

 woody, sometimes to a wonderful degree ; and in some 

 cases they acquire an internal structure like that of the 

 stem or axis. 



The young internodes of the JjOflxospermum as well 

 as the petioles are sensitive to a touch, and by their 

 combined movement seize an object. The flower- 

 peduncles of the Maurandia semperflorens revolve 

 spontaneously and are sensitive to a touch, yet are not 

 used for climbing. The leaves of at least two, and 

 probably of most, of the species of Clematis, of Fumaria 

 and Adlumia, spontaneously curve from side to side, 

 like the interncdes, and are thus better adapted to 



