56 A TEXTBOOK OF BOTANY [Ch. Ill, 7 



The phototropic response of leaves and stems to light, or 

 of any other parts to a stimulus, involves the cooperation of 

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

 property by virtue whereof the plant makes the responses, 

 which are usually adaptive and evidently acquired in evolu- 

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



Fig. 26. — A leaf-mosaic in English Ivy. (After Kerner, Das Pflanzen- 



leben.) 



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

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

 to make a physical impression upon the sensitive proto- 

 plasm. Probably most of the protoplasm 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 arrangements, 

 supposed to facilitate its passage, have also been described. 

 Fourth, there is a motor mechanism, resting usually upon 

 a differential activity in a growth zone or other growing 

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

 Plant and Venus Fly-trap (page 76), a quick-acting hydraulic 



