56 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 7 



The phototropic response of leaves and stems to light, or 

 of an.y other parts to a stimulus, involves the cooperation of 

 four factors. First, there exists in the plant an hereditary 

 property by \-irtue whereof the plant makes the responses, 

 which are usually adaptive ami evidently acquired in evolu- 

 tion in the same way as other plant-features. Second, there 



Fig. 26. 



- A leaf-iiinsair in Eii^lisli Ivy. 



Irhrn.j 



(After Kcrner, Das Pflatuen- 



is some mode of perception of light by the plant, the cjuantity 

 of light needed being extremely small, only enough, indeed, 

 to make a physical impression upon the sensitive proto- 

 plasm. Probatil}^ most of the iirotojjlasm of leaf and stem 

 is thus sensitive, though special regions are more so than 

 others, and various adaptations for concentrating light in- 

 side specialized perception cells have been described. Third, 

 there is some method of transmission of an influence from 

 the perceptive place to a motor mechanism where the actual 

 response is produced. This influence apparently travels, as 

 a rule, through the protoplasm of the cells and the inter- 

 cellular threads (page 40), although special arra.ngements, 

 supposed to facilitate its passage, have also been described. 

 Finnili, there is a, motor mechanism, resting usually upon 

 a differential acti\'it>- in a. grijwth zone or other growing 

 tissue, though in more active responses, as in the Sensitive 

 Plant and Venus Fl.y-trap (page 76), a quick-acting hydrauhc 



