THE MOVEMENTS AND SENSITIVITY OF PLANTS 317 



lings are always more sensitive to light than are green ones. In 

 general, it has been found that growing plants in light decreases 

 their phototropic sensitivity in proportion to the intensity of light, 



Fig. 126. — The leaf mosaic of an ivy (after Lubimenko). 



therefore, plants in open spaces are less sensitive to light than those 

 growing in the shade, while the most sensitive are those which have 

 grown in total darkness. The latter are, as a rule, used for pre- 

 cise phototropic experiments. These experiments are carried out 

 in phototropic chambers, tight 

 boxes having their interior painted 

 black and admitting light through 

 a small opening in one of the walls 

 (Fig. 127). 



Similar to all responses to ir- 

 ritation, the phototropic process 

 consists of four phases: (1) the 

 effect of the stimulus (in the present 



instance, of light) On the Cells; (2) Fig. 127. — Sporangia of Pilobolus 



creation of excitation in the SSSnSKS l^TCf? fm f 



phototropic camera {after smith, et at.). 



the transmission of ex- 



the 



cells; (3) 



citation to the region of reaction; and (4) the response itself, 



which in this instance consists in the formation of a curvature. 



As a rule, only the first and the last phases of the process may 

 be observed. But Darwin (1880) succeeded in proving that in 

 cases of phototropic curvatures it is not merely a question of the 

 direct effect of light on the bending parts. In a number of later in- 

 vestigations, particularly in those by Rothert (1894) a definite 

 subdivision of the process into separate phases was attained. 



