104 GROWTH HORMONES IN PLANTS 



of growth substances upon growth, we have found that they 

 influence cell enlargement ("stretching growth") in diverse 

 kinds of higher plants. The subject is not so simple because in 

 some organs growth is stimulated, while in others it is inhibited 

 by the presence of growth substance. In coleoptiles and portions 

 of stems, growth is increased, while in roots, it is retarded; it is 

 possible that growth substance may not be necessary for root 

 growth, though its role in the neoformation of roots has been 

 observed. Moreover, it appears to have little significance for the 

 vegetative growth of Aspergillus and perhaps many other lower 

 plants. The next step is to determine in what way growth 

 substances exercise their growth-promoting effect. 



From investigations carried out by Went (1928a), van der 

 Weij (1932), and duBuy (1933) on the transport of growth sub- 

 stance, it can be seen that the hormone is actually used up in the 

 growth of the Avena coleoptile. Van der Weij has shown that if 

 two agar blocks, each with the same growth-substance concentra- 

 tion, are placed on either end of a coleoptile cylinder 2 mm. long, a 

 decrease in growth substance takes place in the upper block, but 

 no increase can be demonstrated in the lower block. The most 

 likely explanation of this and numerous similar observations is 

 that it is consumed in growth. 



Growth Substance and the Cell Wall. — Up to the present, 

 close quantitative relationships between consumption of growth 

 substance and growth have not been demonstrated, and the 

 present evidence is insufficient to prove that it participates 

 stoichiometrically in the growth of the cell wall. Nielsen (1930a, 

 h) showed that its effect is very great in proportion to its weight; 

 and according to Kogl (1933, Mitt. Ill; see also Kogl, 1933a), a 

 curvature of 10 deg. results in the Avena coleoptile from the 

 action of less than one 50-millionth milligram of auxin a or b. 

 Thimann and Bonner (1933) have computed that 2.31 X lO^i 

 growth-substance molecules can produce a deposition of 

 6.8 X 10^^ CeHioOs molecules of glucose residues for the cellulose 

 micelles, i.e., that one growth-substance molecule is active in 

 the formation of 3.0 X 10^ CeHioOs molecules. Although these 

 numbers are approximations, they are entirely adequate to show 

 that growth substance does not participate as a "building stone" 

 of the cell wall ; it must influence the growth of the cell in some 

 other way. At this point in the discussion, it may be well to 



