190 CHEMICAL AGENTS AND GROWTH 



Although these resuhs indicate that GA affects some of the responses 

 also affected by red and FR, all the responses in question are promoted 

 by GA, so that in some cases (seeds, leaf discs) it acts in the same 

 direction as red light, while in others (internode elongation), in the 

 same direction as FR. Any hypothesis of a close relationship between 

 GA and the photomorphogenic system needs to explain this circum- 

 stance. 



Turning to the results at hand, three summary statements can be 

 made. ( 1 ) Qualitatively, the response of pea internode sections to red 

 and FR depends on the previous light regime of the plants used. (2) 

 Quantitatively, the red response depends on the endogenous growth of 

 the tissue exposed. (3) The presence of exogenous GA or auxin has 

 no effect on either red or FR response, with the exception of high 

 auxin levels, which prevent both. The first statement was illustrated 

 in Fig. 1 ; similar observations have been made on various materials 

 by others. The second point, however, deserves further consideration. 

 The data in Fig. 2 suggest that red light inhibits endogenous growth, 

 and endogenous growth only: the inhibition was much greater in the 

 rapidly growing apical (A) sections than in the basal (B) sections, 

 but when the growth of the latter was increased by lAA, no change in 

 the amount of red inhibition occurred. This leads to a more extended 

 consideration of the third statement. 



The amount of inhibition caused by red light is approximately the 

 same whether GA is present or not (Fig. 3, Dg) ; in tissues which are 

 not sensitive to red light, the increased growth due to GA remains 

 insensitive to red light (Fig. 3, Rg). Similarly, the amount of elonga- 

 tion caused by FR is approximately the same in the presence or 

 absence of GA (Fig. 4, Rg); and tissues insensitive to FR remain so 

 in the presence of GA (Fig. 4, Dg). For similar resuks with GA and 

 FR on intact plants see Downs et al. (1957). Statements similar to 

 those on GA above can also be made for lAA in concentrations thru 

 10-^M, and are illustrated by the data in Figs. 2, 5 A, and 6. All these 

 results agree with the concept that what is inhibited by red light is a 

 portion of endogenous growth, that this component is already com- 

 pletely repressed in sections from red-grown plants, and that neither 

 lAA-induced growth nor GA-induced growth is red sensitive. The 

 converse would be true for FR action. 



