112 HISTOLOGY AND PHYSIOLOGY. 



bearing plants, may pass their entire existence in dark- 

 ness. The degree and kind of light influences the amount 

 of assimilation. There is here, as in ease of temperature, 

 a minimum, optimum, and maximum intensity. If the 

 amount of assimilation in white light be taken as 100, 

 that for each of the isolated rays of the solar spectrum 

 is found on the average to be as follows : Red, 9.5 ; 

 Orange, 23.5; Yellow, 27.3; Green, 14; Blue, 8.2; 

 Indigo, 5 ; Violet, 2.5. 



The more refrangible rays are less efficacious; from 

 the yellow and orange rays there is a decrease in both 

 directions ; and in the heat and actinic rays, found respect- 

 ively beyond the red and violet, no assimilation whatever 

 takes place. 



MOVEMENTS. 



143. If all other conditions are made constant, the 

 rapidity of growth of most aerial stems is greater in dark- 

 ness than in light. This is due to the retarding influence 

 of the rays of high refrangibility (blue, indigo, violet, and 

 ultra-violet). When, therefore, the illumination is greater 

 on one side of the stem than on the other, a curvature 

 arises in consequence of the retarding influence of light. 

 Thus, when plants are grown in windows, they curve 

 strongly towards the light. This phenomenon has received 

 the name of heliotropism (Gr. helios, sun ; trepein, to 

 turn). It is due to the fact that the growth of the cells on 

 the illuminated side is retarded, while on the opposite side 

 the cells elongate, causing a curvature. Some organs, 

 however, bend away from the light, indicated by the terra 

 negative heliotropism (the former being positive 

 heliotropism), the explanation of which is not as yet 

 clear. Thus the tendrils of the Grape- Vine and Virginia 



