INTERACTION OF GROWTH FACTORS WITH 

 PHOTOPROCESS IN SEEDLING GROWTH' 



WILLIAM H. KLEIN 

 Smithsonian Institution, Washington, D. C. 



Test Object 



During the germination of a bean seedling a hook develops in the 

 hypocotyl. As growth proceeds in the dark, the hook moves up into 

 the epicotyl. In total darkness this hook does not completely disappear 

 by the time the food reserves are depleted. However, when exposed to 

 low values of irradiance in the red between 600 and 700 mju, the hook 

 opens and completely disappears in several days. This hypocotyl hook 

 can be excised and used to determine quantitatively the photomorpho- 

 genic response (Klein et al., 1956). 



Histology of Dark and Red-Exposed Hooks 



Red-treated and control hooks (dark) were fixed with formalin- 

 aceto-alcohol and embedded in paraffin. Sections were cut longi- 

 tudinally 10 ;ti in thickness and stained. Since no stage of cells under- 

 going mitosis has been detected in any of the numerous prepared 

 slides, it can be assumed that the growth response is due to cell elonga- 

 tion. The degree of hook opening depends upon the relative growth 

 rates of the inner and outer sides of the hook. The cells on the inner 

 (concave) side are stimulated to elongate by the red treatment, while 

 the cells on the outer side (convex) are relatively unaffected. The 

 elongation is proportional to the incident energy. This work was done 

 in cooperation with Dr. C. C. Moh. Figure 1 is a photomicrographic 

 drawing of the inner and outer sides of the dark-treated and red- 

 treated hooks. 



1 Published with the approval of the Secretary of the Smithsonian Institution. 



207 



