194 CHEMICAL AGENTS AND GROWTH 



endogenous growth in both types of sections. It also appears to be an 

 effective inhibitor of lAA-induced elongation (Fig. 5B) and, in similar 

 experiments, GA-induced elongation. Hence its effect on elongation 

 cannot be referred to any of the individual "components" described. 

 Thomson (1954) studied the effects of occasional (white) light 

 treatment on etiolated pea and Avena seedlings. She concluded that 

 "Exposure very early in the course of either cell division or cell elonga- 

 tion accelerates the early part of the phase of growth. Exposure after 

 either phase is well under way hastens the transition to the next phase 

 and reduces the final number or length of cells." In short, light hastens 

 maturation. The relevance of this work to the present results is that 

 the "components" discussed above may simply represent phases in 

 internode development, arbitrarily isolated from the developing organ- 



ism at a given time. 



On this basis, red inhibition of elongation in an intact plant might 

 also result in increased sensitivity to a growth factor such as GA 

 (Lockhart, 1956), if the light accelerates the development to a phase 

 requiring that growth factor. In this regard it is significant that apical 

 sections, which show a high endogenous growth and red sensitivity, 

 are also more responsive to GA than more basal sections (Purves and 

 Hillman, 1958), ahhough no direct relation between endogenous 

 growth and GA response could be established. As sheer speculation, it 

 is possible to suppose that the development of very young internode 

 tissue passes through at least three stages: (1) endogenous growth, 

 highly red sensitive; (2) a stage limited by GA; (3) a stage limited by 

 auxin. Thus the light-induced increase in GA response found in intact 

 plants would be due to the fact that cell maturation proceeds un- 

 hindered, which would not be true for sections. 



It should be clear that the intent of the scheme presented in Fig. 7 

 and of this discussion is not to propose a mechanism for the interaction 

 of radiations and growth substances; on the contrary, it is to show 

 how "interactions" might be observed even if red-FR control is exerted 

 on completely unknown processes. Admittedly, the results of Liverman 

 and Bonner (1953) remain unexplained by this concept, perhaps 

 because coleoptile tissue is somewhat anomalous in being promoted 

 by both auxin and red light. However, the results of Schneider ( 1941 ), 

 Thomson (1954), and those reported here, as well as the general con- 



