54 PLANT RESPONSE 



ordinary leaves are obviously anisotropic, their upper and 

 lower halves being quite unlike. We might, therefore, expect 

 to find in them the exhibition of differential response. There 

 is, however, some difficulty in detecting these responsive 

 movements of ordinary leaves in an unmistakable manner, 

 inasmuch as flexibility is not so great in ordinary petioles 

 as in those which are provided with a pulvinus. Moderate 

 stimulus therefore causes in these relatively smaller move- 

 ments, and unless some form of stimulation can be used 

 which brings about no mechanical disturbance of the plant 

 itself, it is impossible to discriminate the true responsive 

 movement. I have, however, described modes of stimula- 

 tion, by the electro-thermic stimulator and by electric 

 shocks, by means of which this difficulty is overcome. By 

 the use of the Optic Lever, further, a magnified record 

 of the responsive movement and its time-relations may be 

 obtained. 



Pulvini proper and pulvinoids. — We have seen that the 

 responsive effect is caused by the turgidity-variation due to 

 stimulus. Such motile indications can occur with facility 

 only in tissues which are not yet hardened. We may there- 

 fore expect to find motile effects throughout the anisotropic 

 petiole and its prolongations, especially in young leaves. 

 After a certain time, in many instances, it is only portions of 

 the petiole, such as those at the junctions of the petiole with 

 the stem and lamina, that remain flexible. These flexible 

 points are sometimes rather swollen or cushion-like, and may 

 be seen — though in a nmch less developed condition than in 

 Mimosa — in the leaves of many ordinary plants. But such 

 areas, in ordinary leaves, are usually regarded as non-motile, 

 and therefore functionally distinct from the true pulvini in 

 sensitive plants. I shall therefore, for the sake of convenience, 

 distinguish between pulvini proper and these pulvinoids. But 

 I shall show presently that, contrary to the usual belief, these 

 pulvinoids also are fully sensitive. Functionally, then, we 

 have in young leaves a diffuse pulvinoid, which is capable of 

 mechanical motility, throughout the length of the petiole. 



