652 PLANT RESPONSE 



dition of the leaflet during the ten minutes' exposure of the 

 lamina to the action of light, and its energetic movement 

 immediately on the application of light to the pulvinus, 

 shows once more, in a striking manner, that it is the latter 

 organ and not the lamina whose perception of light is effec- 

 tive in initiating the responsive action. 



Principal types of the response of leaves to stimulus 

 of light. — I shall now proceed to show that the directive 

 effect of light on leaves is very definite. In studying the 

 response of anisotropic organs — that is to say, plagiotropic 

 shoots and dorsi-ventral pulvinated leaves — we have seen 

 that there are two extreme types of response, of which the 

 first is exhibited by organs possessing feeble transverse con- 

 ductivity, and the second by those in which the transverse 

 conductivity is great, and the lower side the more excit- 

 able. In the first of these types, light acting on the organ 

 from above gives rise to a positive heliotropic response. In 

 the latter, long-continued application of strong light from 

 above gives rise to internal diffusion of stimulus, causing 

 concavity of the more excitable lower half, with the result of 

 negative heliotropic response. Intermediate between these 

 we have seen that there are cases of the equal excitation ot 

 the upper and lower halves of the organ, bringing about the 

 so-called dia-heliotropic position. As examples of the two 

 extreme types — within which lie the responses of all ordinary 

 leaves — I shall here give records of the movements of leaves 

 of Mango (^JMangifera indica) and o{ Artocarpiis. 



Positive type of response : Mango. — The leaves of 

 this plant when young are bent abruptly downwards by the 

 sharp curvature of the short petiole. In the course of a week 

 or so, however, they rise, and gradually attain a position at 

 or above the horizontal, a process in which the action of 

 light is an important agent. This will be seen from the 

 following record of the movement of a Mango leaf when 

 acted upon by sunlight from above (fig. 263). The record 

 before the application of light was practically horizontal, 

 showing that there was little natural movement ; but the 



