GROWTH SUBSTANCES-RED LIGHT INTERACTIONS 195 



sideration that red and FR control diverse processes in botli plants 

 and animals, make it seem unlikely that this control is exerted in any 

 but the most indirect manner through interactions with plant growth 

 substances such as lAA or GA. 



The conclusion that photomorphogenesis is not mediated by known 

 growth substances suggests that a much closer examination of endoge- 

 nous growth in both excised tissues and intact plants should be under- 

 taken, preferably coupled with anatomical and cytological studies. If 

 a guess were to be made as to the kind of system which might be 

 involved in photomorphogenesis, one affecting purine metabolism 

 might be suggested. Purines and related compounds have been im- 

 plicated in photomorphogenesis by several groups (cf. Scott and Liver- 

 man, 1957), and their basic role in nucleic acids and energy transfer 

 at least makes it possible to conceive that a system affecting them 

 might in turn control overall cell maturation and differentiation in 

 many diverse ways. 



SUMMARY 



The growth response of pea seedling internode sections to red or far- 

 red (FR) radiations depends upon the previous light regime of the 

 plants used. Red light inhibits elongation in sections from completely 

 dark-grown plants, but it has little or no effect on sections from plants 

 exposed to red light during the preceding 18 hr (red-grown plants). 

 Elongation of sections from red-grown plants is promoted by FR, 

 but sections from dark-grown plants are not affected. For sections 

 from dark-grown plants, the greater the endogenous growth (no added 

 growth substances) the greater is the inhibition by red light. 



Neither the direction nor the absolute magnitudes of the red or FR 

 responses are affected by gibberellic acid (GA) or auxin (lAA), 

 except in the case of high lAA, which prevents both responses. 

 Kinetin, unlike red hght, inhibits the elongation of both types of 

 sections equally, and is an effective inhibitor of lAA- and GA-induced 

 growth. 



These results suggest that the radiations act primarily on a com- 

 ponent of endogenous growth which is distinct from lAA- or GA- 

 induced growth. Some implications of this view are discussed. 



