GROWTH SUBSTANCES FOR NORMAL GROWTH 93 



of the factor complexes significant in its growth. Went has 

 discussed the subject in a later paper (1935c) and concluded that 

 growth substance is a limiting factor in the elongation of the 

 coleoptile during its later stages of development. Artificially 

 increasing the auxin supply in a coleoptile accelerates the 

 growth rate either directly by promoting growth or indirectly 

 by preventing senescence (Fig. 29). With a supply of food 

 available, the addition of auxin brought about a revival of growth 

 in the basal portion which had ceased to elongate; on the other 

 hand, when the food supply was removed, further additions of 

 auxin showed no growth-promoting effect. 



From the foregoing observations it seems clear that the rate 

 and distribution of growth in the normally developing coleoptile 

 are regulated by the supply of growth substance. 



Growth Substances and the Light-growth Reaction. — Went (1926) 

 and van Dillewijn (1927a) conjectured that the growth reactions 

 produced by complete illumination of the tip zone are the result of 

 changes in the amount of growth substance given off, and Went 

 (1928a) actually found a decrease of about 18 per cent in the 

 amount of auxin given off when the tip was illuminated with 

 1,000 meter-candle seconds. According to duBuy (1933), weak 

 blue light produces no change in growth-substance supply, and 

 even strong white light (with heat and some of the red removed) 

 may have no effect; on the other hand, white light plus the heat 

 radiation decreases the supply of growth substance. 



General illumination of the lower zones of the coleoptile also 

 produces light-growth reactions, as mentioned earlier, but it is 

 not possible at this time to offer a satisfactory explanation of the 

 phenomenon. Further data are discussed luider light-growth 

 reaction in the stems of seedlings. That the influence of light 

 on growth depends upon the kind of growth substance present 

 has been shown by van Overbeek (1936a). When auxin a is 

 applied unilaterally in agar blocks to Avena coleoptiles, curvature 

 is less under illumination with white hght, than in darkness; 

 when 3-indole acetic acid is similarly applied, no difference in 

 growth is observed in darkness or in light. 



Foliage Leaves. — The presence of growth substance has been 

 demonstrated in buds and foliage leaves of several species of 

 dicotyledons (see chapter on the occurrence of growth sub- 

 stances), and in Nicotiana (Avery, 1935) it has been shown that 



