568 PLANT RESPONSE 



periodic movement is held to be dependent on paratonic 

 effects and phototonus. The heliotropic effect is ascribed to 

 the continuous action of light, independent of phototonus. 



Thus in the same organ we have to postulate various 

 irritabilities and mechanisms, in order to account for its 

 multifarious movements. Are there, then, independent or 

 different irritabilities, coexisting simultaneously in the same 

 organ ? Such a state of things is so difficult to imagine, that 

 it prompts us to try to look at the problem in a fresh light, 

 divested of all assumptions which are incapable of experi- 

 mental proof. 



Approaching the matter thus directly, then, we see that, 

 instead of so many different irritabilities, there may possibly 

 be, fundamentally, but a single phenomenon of irritability, 

 finding expressions apparently diverse, in consequence of the 

 anatomical or physiological differentiations of the responding 

 organ. If this should be so, the question will resolve itself 

 into three separate inquiries. First, what is that responsive 

 action which constitutes the characteristic effect of stimulus 

 of light ? Second, is such response to light unique in 

 character, or is it a single instance of that universal 

 phenomenon of contraction which we have seen to be the 

 response of all excitable cells to stimulus in general ? And, 

 lastly, in what manner do the various anatomical and physio- 

 logical differentiations of responding organs operate to modify 

 the expression of this response ? 



Action of light on tissues in sub -tonic condition. — Before 

 proceeding to a decisive demonstration of the nature of the 

 effect of light on excitable tissues in a normal condition, 

 however, I shall briefly refer to the suggestion which has 

 been offered, that light in some unknown way induces a 

 lessening of turgor, which brings about diminution of growth. 

 This theory could not hitherto find acceptance for want of a 

 precise knowledge of the exact nature of the stimulatory 

 action of light, and of the relative significance of absorbed 

 energy in promoting growth. Against the assumption that 

 light diminished turgor, it was urged that the pileus of 



