192 



CHEMICAL AGENTS AND GROWTH 



FR-induced endogenous growth in sections from red-grown plants is 

 completely suppressed by red, while lAA-induced growth in the same 

 material is unaffected. Exactly parallel arguments will also suggest 

 that GA-induced srowth and endogenous growth are not identical. 

 Thus the postulation of a light-sensitive endogenous growth component 

 of unknown nature appears to offer fewer difficulties than the tradi- 

 tional assumption. This is not to say that no portion of endogenous 

 growth is lAA-mediated; there is a great deal of evidence against such 

 a position. It is only necessary to assume that a portion of it is not 

 lAA-mediated, and that this is the portion controlled by red light. 

 This interpretation is in complete agreement with the results of 

 Schneider (1941), who pointed out that "the effect is much as though 

 [red] light caused a given absolute amount of inhibition, the amount 

 being nearly the same under all conditions of auxin supply." 



A diagram summarizing this concept is presented in Fig. 7, in which 



Fig. 7. Summary of effects of various agents on elongation of pea stem 

 sections. See Discussion. 



the pointed ends of the figures indicate promotion or inhibition of 

 elongation depending on their direction relative to that of the central 

 figure. Figure 7 represents elongation in excised sections as contributed 

 to by (at least) three more or less independent components: endoge- 

 nous growth, lAA-induced growth, and GA-induced growth. The 

 separation of lAA- and GA-induced growth is based primarily on the 

 results of Purves and Hillman (1958) for the same material, but also 

 on the behef that it is better to assume a lack of any direct relation 

 until there is unequivocal evidence of interaction. Red and FR are 



