MECHANICAL RESPONSE IN ORDINARY LEAVES 6 1 



It will thus be seen that the petiole when young is both 

 the motile and the transmitting organ. With increasing 

 age, certain areas become inflexible, and the power of 

 motility is narrowed to the still remaining points of flexi- 

 bility — the pulvinoids. But the inflexible petiolar portion 

 still retains the power of transmitting stimulus. Using the 

 language of Animal Physiology, we might say, therefore, 

 that a young petiole is, functionally, nerve and muscle 

 combined. Later there is a differentiation into motile pulvi- 

 noids, corresponding to muscle, and the rest corresponding 

 to conducting nerve. The pulvinoid, however, has still the 

 power of transmitting stimulus. 



To serve the purpose of a concrete example, I have 

 taken as a typical case the Artocarpus leaf. I shall now, 

 however, show that responsive motile effects are obtainable 

 from the leaves of plants in general. In the record given 

 (fig. 32) I applied only moderate stimulus, in order that the 

 magnified response might be brought within the limited 

 space of record. From this, it might be supposed that the 

 extent of responsive movement in ordinary leaves is rela- 

 tively much smaller than what is obtained with the sensitive 

 Mimosa. But this is by no means the case. By using 

 stronger stimulus a response is obtained, in the case of many 

 ordinary leaves, which in its extent is strictly comparable 

 with that of Mimosa. Of this, I shall here proceed to give 

 examples. But, before doing so, we must remind ourselves 

 that the promptitude of mechanical response is a matter of 

 the delicacy of poise of the motile apparatus. In the case of 

 Philanthus, which is provided with distinct pulvini, we found 

 that response took place very sluggishly, and was evoked 

 only by relatively strong stimulus. The difference between 

 the response in this case and in that of Mimosa is, however, 

 not of kind, but of degree. 



Response of ordinary leaves comparable with that of 

 Mimosa. — When we come to the response of ordinary leaves, 

 we find, under strong stimulation, considerable extents of 

 movement, even rivalling those of Mimosa, but taking place 



