MECHANICAL RESPONSE IN ORDINARY LEAVES 65 



under continuous stimulation exhibit complete, and some 

 partial, reversals, there are again other species of plants whose 

 leaves do not exhibit any reversal at all. Still others, again, 

 exhibit periodic reversals. The effects produced are thus 

 seen to depend on the species of the plant, on the age of the 

 leaf, and on the intensity and duration of the stimulus. 



It will thus be seen that an anisotropic organ like the 

 petiole responds to stimulus by differential contraction of its 

 upper and lower halves, and that the mechanics of such 

 movements in the case of ordinary leaves are precisely the 

 same as in those of the leaves of sensitive plants such as 

 Mimosa. In the next chapter I hope to show that even 

 radial organs are not insensitive, but exhibit responsive con- 

 tractile movements. 



Summary 



The motile organs of ordinary leaves, owing to dorsi- 

 ventral differentiation, give mechanical response by differential 

 contraction, in a manner precisely the same as does the 

 motile organ oi Mimosa. 



In a young leaf, the petiole and its prolongation act as a 

 diffuse pulvinoid ; later, the motile area becomes restricted 

 to certain points, generally speaking, the junctions of petiole 

 and stem, and petiole and lamina. There is no sharp line of 

 demarcation between pulvinoids and pulvini proper. 



Even in relatively old and rigid tissues, response to stimu- 

 lus is by excitatory expulsion of water. 



Responsive movements take place in ordinary leaves, 

 when the stimulus is applied at or near the motile organ. 



The lamina is not, generally speaking, the perceptive 

 organ. The effect of stimulus remains in this case localised, 

 and is not transmitted to a distance. 



Organs like the petiole, which contain fibro-vascular 

 elements, are capable of conducting stimulus to a distance. 



Under continuous stimulation there may be reversal due 

 to fatigue in ordinary leaves as in Mimosa. In some cases, 

 periodic reversals are also observed. 



E 



