88 GROWTH HORMONES IN PLANTS 



coleoptile in darkness and when subjected to varying amounts of 

 light. As may be seen in Fig. 27, he found that the rate of growth 

 of the coleoptile is temporarily increased with increasing amounts 

 of light. The point of maximum growth is reached sooner, and 

 the final size of the coleoptile is not so great under conditions of 

 increasing light. 



Van Dillewijn (1927a) illuminated the Avena coleoptile by 

 placing a lamp vertically over the plant; the hght was reflected 

 horizontally on to the experimental object by three obHque 

 mirrors. He noted the influence upon the rate of growth of 

 continued illumination as well as short periods of illumination 

 with differing amounts of light. In some experiments only the 

 tip was illuminated, in others, the subapical zones; or the entire 

 coleoptile was supplied with hght. Light-growth reactions 

 appeared in all cases. The reactions were sharply defined when 

 definite zones, near the tip, were illuminated for a short time with 

 a definitely determined amount of light. After a latent period 

 there occurred a depression of growth during the course of which 

 two types of response could be distinguished, one of short, the 

 other of long duration (Sierp, 1921). In the short reaction, the 

 maximum depression of the growth rate was reached after 

 }/2 hour; in the long reaction, on the other hand, after 1}4 hours. 

 The long reaction could be observed only when the tip was 

 illuminated (Went, 1928a: tip response); the short reaction, when 

 the basal zones were illuminated (Went: base response). Growth 

 was accelerated again after tliis retardation. When the entire 

 coleoptile was illuminated, these effects were summated. When 

 illuminated plants were darkened, Sierp (1918) found that a dark- 

 growth reaction took place also. 



Geo-growth Reaction. — The question whether gravity can 

 produce fluctuations in the growth of the Avena coleoptile in a 

 manner similar to that brought about by light has been investi- 

 gated with contradictory results. Zolhkofer (1921) reported 

 continually changing rates of growth in response to stimulation 

 by gravity; while Koningsberger (1922) observed no geo-growth 

 reaction during continued rotation on the clinostat, but a growth- 

 promoting effect was produced by gravity in both erect and 

 inverted coleoptiles. The dorsiventrality curvatures, however, 

 mentioned elsewhere, can easily disguise growth changes when the 

 Avena coleoptile is cUnostated. With this source of error 



