GROWTH SUBSTANCES-RED LIGHT INTERACTIONS 



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Fig. 6. Elongation of 8-mm sections cut 4 mm from apex of red-grown 

 (Rg) or dark-grown (Dg) plants, as affected by red or far-red treatment 

 at indicated lAA levels. (Rg exp. 5-9-57; Dg exp. 5-15-57.) 



A concept that calls for a qualitative distinction between endoge- 

 nous growth and that induced by growth substances, particularly lAA, 

 is admittedly disturbing. The assumption that lAA-induced growth is 

 identical and coextensive with endogenous growth is implicit in most 

 of the literature on section growth. While this general question cannot 

 be discussed here, the writer is unaware of any systematic attempts 

 to compare endogenous growth to lAA-induced growth with respect 

 to inhibitor sensitivity, for example, or any other characteristic, and 

 the assumption has remained an assumption. It is difficult to maintain 

 against the present evidence. 



Endogenous growth is clearly red sensitive in sections from dark- 

 grown plants (Figs. 1, 2). If it is identical with lAA-induced growth, 

 then as 2rowth is increased with lAA the red inhibition should increase 

 in proportion, which it does not do. If all endogenous growth is 

 identical with lAA-induced growth, it is difficult to explain why the 



