270 LIFE MOVEMENTS IN PLANTS 



One of the causes of uncertainty lay with the question, 

 whether response changed with the mode of stimulation. I 

 have, however, been able to show that all forms of stimuli 

 induce a definite excitatory reaction of contraction (p. 218). 



Tropic movements induced by unilateral action of 

 stimulus may, broadly speaking, be divided into two classes 

 depending on the point of application of stimulus : 



In the first, the point of application of unilateral 

 stimulus is not on the responding organ itself, but at 

 some distance from it. The question therefore relates to 

 Longitudinal Transmission of ett'ect of stimulus. 



In the second, unilateral stimulus acts directly on the 

 responding organ. For the determination of the resultant 

 movement, it is necessary to take account of effects 

 induced on the two sides of the organ. The side adjacent 

 to the stimulus I shall designate as the proximal, and 

 the diametrically opposite as the distal side. The question 

 to be investigated in this case relates to TRANSVERSE 

 TRANSMISSION of effect of stimulus. It will be shown that 

 the resulting movement depends on : — 



(a) whether the tissue is a conductor or a non- 

 conductor of excitation in a transverse direc- 

 tion, and 



(h) whether it is the proximal, or the distal side of 

 the organ that is the more excitable. 



In connection with the response to environmental 

 changes, a source of uncertainty is traceable to the absence 

 of sufficient knowledge of the physiological effect of heat, 

 which has been regarded as a form of stimulus: it will 

 be shown that heat induces two distinct eflfects dependent 

 on conduction and radiation. We shall in the succeeding 

 chapters, take up the study of the physiological effects 

 induced by changes in the environment. 



