186 CHEMICAL AGENTS AND GROWTH 



from dark-grown and red-grown plants. GA alone promoted elonga- 

 tion, as usual. FR alone, as shown previously, promoted elongation 

 only in sections from red-grown plants. GA and FR together had the 

 same effect as GA alone on sections from dark-grown plants, but on 

 sections from red-grown plants their effects were almost perfectly 

 additive. 



Interactions of Indoleacetic Acid with Red and Far-Red Treatments 



Galston and Baker ( 1953) found that the optimum lAA concentra- 

 tion for the elongation of sections from red-grown plants was about 

 100 times that for sections from dark-grown plants. This was easily 

 confirmed. With 8-mm sections cut 4 mm below the apex, an elonga- 

 tion optimum ca. lO^^^M (0.175 mg/1) lAA was found for sections 

 from dark-grown plants as compared with ca. lO^M for the red- 

 grown. Almost identical results were also obtained with naphthalene- 

 acetic acid (NAA). Fresh weight increase, however, was maximal for 

 sections from dark-grown plants at ca. 10~''M lAA or NAA, (cf. 

 Galston and Hand, 1949) whereas for sections from red-grown plants 

 it was only slightly higher at {0~^M than at 10~'^M. Thus the differ- 

 ence in auxin sensitivity between the two types of sections is much less 

 marked when considered in terms of fresh weight increase. 



It was already shown that red treatment inhibits elongation in sec- 

 tions from dark-grown plants in the absence of lAA. It also inhibits at 

 all levels of lAA up through 10 "M; the absolute value of the inhibi- 

 tion is approximately constant regardless of the lAA level, so that 

 there is a decrease in percentage inhibition with increasing lAA. 

 Figure 2 provides two examples of this. In Fig. 2A, the difference 

 between the control (C) and red treatment (R) is the same as that 

 between the 10 "M lAA treatment (i) and its red-treated counterpart 

 (iR). This level of lAA is optimal for the elongation of the 5-mm 

 apical sections used (Purves and Hillman. 1958). In Fig. 2B, again, 

 the difference between the control and red treatment is the same as that 

 between the IQ-'^M lAA treatment (I) and its red counterpart (IR). 

 It is also worth noting that, while the higher red sensitivity of the 

 apical sections (Fig. 2A) appears to be correlated with a higher 

 endogenous growth, the increased elongation of the basal sections 

 caused by lAA did not confer a proportional increase in red sensitivity. 



