184 



CHEMICAL AGENTS AND GROWTH 



(FR) treatments given to the sections themselves on the endogenous 

 growth of sections from dark-grown and red-grown plants. Red 

 inhibited the elongation of sections from dark-grown plants; FR had 

 little or no effect by itself (occasionally causing a slight inhibition), 

 but it reversed the red inhibition when given directly after the red 

 treatment. The elongation of sections from red-grown plants was not 

 affected by red treatment, but was promoted by FR; the FR promotion 

 was reversed by subsequent red light. 



A relationship between the red inhibition and endogenous growth is 

 indicated in Fig. 2, representing an experiment conducted with 5 -mm 



B 



z 

 o 



LJ 



R 



R 



Fig. 2. Elongation of 5-mm sections from dark-grown plants, cut 1 

 mm (A) or 6 mm (B) from apex, as affected by red light treatment (R) 

 in the presence or absence of optimal lAA. i, 10~''M I A A; I, 10~^M 

 lAA. (Exp. 9-12-57.) 



sections taken either 1 or 6 mm from the apex of dark-grown plants. 

 Endogenous growth in the apical sections (Fig. 2A: C) was much 

 greater than in the basal sections (Fig. 2B: C) ; the red-light inhibition 

 was also greater, not only in absolute terms but also proportionally. 

 The effects of indoleacetic acid (lAA) also shown in Fig. 2 will be 

 discussed later. 



Interaction of Gibberellic Acid witli Red and Far-Red Treatments 



Elongation in sections from both dark-grown and red-grown plants 

 is promoted by gibberellic acid (GA) at concentrations from 10^ to 

 10~*M, with what appears to be a weakly marked optimum at 10 ''M 

 (0.346 mg/1). Figure 3 shows some interactions of red treatment and 

 10~^'M GA on sections from both types of plants. GA promoted 

 elongation in both types of sections. Red light inhibited the elongation 



