896 LIGHT AND LIFE 



jnoduce a marked curvature (although the coleoptiles otherwise 

 would bend very little after such a treatment) . This, to Thimann, 

 suggests that the light does not react with the auxin itself but with 

 some substance capable of lasting a considerable time in the plant and 

 of "influencing many hundreds of auxin molecules." Although an 

 enzyme might conceivably do this, it is hard to see how an enzyme 

 might affect the translocation of auxin. Thimann prefers to suppose 

 that the carotenoid proposed as the photoreceptor substance may be 

 located within a plastid, or proplastid, in the coleoptile. He then 

 develops a unified theory of geotropism and phototropism by suggest- 

 ing (a) that the "statoliths" responsible for geotropism are the same 

 proplastids, (b) that the movement of auxin from cell to cell is pro- 

 moted by the presence of the statoliths lying against a particular 

 wall of the cell, and (c) that unilateral light causes the plastids to 

 migrate to the lateral wall opposite to the direction of the light. This 

 theory may also unify the activity of the proplastids and the move- 

 ments of chloroplasts, which likewise are related to light intensity, 

 spectral characteristics, and preliminary treatment with red light. 

 In one respect, however, geotropic and phototropic responses are 

 different. Gravity is detected by the whole coleoptile, not just by the 

 tip. Hence, if the plastids are indeed responsible for both phenomena, 

 those in the tip region must differ from those below. This matter 

 is receiving investigation. 



A. W. Galston and R. Kaur have presented some interesting com- 

 parisons of the effects of auxin and gibberellic acid, applied separately 

 or together, on etiolated and green sections of stem from the garden 

 pea. Sections of stem 3-5 mm long were found to be optimal for test- 

 ing these effects in both green and etiolated plants. The experiments 

 revealed that low doses of red light markedly inhibit the growth on 

 sucrose of etiolated sections, but do not so inhibit green segments; 

 and that the growth of the former is markedly enhanced by cobaltous 

 ion (10~* M) , whereas that of the green sections is not. The etiolated 

 sections respond optimally to low concentrations of gibberellic acid 

 and auxin (K)-" M or less) , but green sections require concentrations 

 100 to 1000 times as great. The stimulation of growth by gibberellic 

 acid and auxin is simply additive in etiolated sections, but is syner- 

 gistic in green sections, especially in plants grown inider an 8-hour 

 photoperiod. The synergism is manifested after as little as 1 minute's 

 exposure to lO^-* M gil)bercllic acid. Substituted ureas used as herbi- 

 cides (CMU and feiunon) also act differentially on the green and 

 etiolated stem segments. They will inhibit the growth of the green 



