434 PLANT RESPONSE 



Effect of direct application of stimulus on the grow- 

 ing region. — Many of the phenomena of growth-curvature 

 are brought about, as we shall see, by means of the changes 

 induced by external stimulus in the rate of growth, and there 

 is much misconception as to whether the effect of stimulus is 

 to enhance or to retard growth. I shall be able to show that 

 this misconception is the result of the complexity of the 

 problem, depending (i) on the tonic condition of the tissue, 

 (2) on whether the stimulus is internal or external, and (3) 

 on the point of application of stimulus. The fundamental 

 effect of stimulus is, however, very definite. 



But at the beginning of our investigation we are met by a 



question of great importance, namely, as to whether the effect 



of stimulus on a growing, is in any 



way different from that on a stationary, 



organ. We have seen, for example, 



that a fully grown style of Datura, in 



which growth has come to a stop, 



Fig. 177. Photographic exhibits on the application of local 



Record of Responses of stimulus the usual contractile response 



Mature Style of Datura A 



alba to External Thermal (fig. 1 77). On the completion of re- 



Stimulus covery again, the organ returns to its 



original length. Hence the base-line of a series of these 

 responses is horizontal. 



I shall now pass, by means of intermediate links, from 

 this to the response of growing organs. And first I shall take 

 the response of a style of Datura, in which, for want of a 

 sufficient supply of internal energy, growth has come to a 

 temporary stop. On applying thermal stimulus, at intervals 

 of a minute, the first five responses are seen to be practically 

 like those of the stationary style of Datura (fig. 178, cf. 177). 

 But a portion of the stimulus applied is being absorbed and 

 held latent in the organ, thus increasing the internal energy, 

 or tonic condition. The result of this is seen in the renewal 

 of growth at the sixth response. The stimulus now, therefore, 

 finds bifurcated expression in maintaining response, and in 

 renewing growth, as is seen by the trend downwards of the 



