98 



GROWTH HORMONES IN PLANTS 



the addition of growth substance. The rate of elongation of 

 decapitated flower stalks is increased by rhizopin, also (Nielsen, 

 1930a). Soding (19326) showed that unilaterally applied Avena 

 tips produce curvatures in flower stalks of Heliopsis laevis, 

 Cephalaria tatarica, and Muscari ramosum; therefore, in these 

 organs, also, the rate of growth is increased by growth substance. 

 That the growth hormone present in the young internodes and 

 nodes of grasses influences their elongation may be concluded 

 from the fact that growth substance from coleoptile tips increases 



4) 



S18 + 



3 



XT. 



10 



c 



lo 



«18- 









E 



Fig. 34. — Comparison of the growth-hormone content and growth response 

 of a normal race of Zea mays with the dwarf type "nana." A, coleoptile tips 

 of the normal race yield almost double the amount of growth hormone given 

 off by the dwarf "nana"; data from 400 plants of each type. B, growth curva- 

 ture of coleoptiles of the normal race is about twice that of "nana" when a given 

 amount of growth hormone is applied unilaterally in agar blocks; data from 

 200 plants of each type. (After van Overbeek, 1935.) 



the growth of young internodes. It has been shown to produce 

 cell elongation even in mature nodes (Schmitz, 1933). 



The first internode (mesocotyl) in Zea mays, dwarf variety 

 "nana," is appreciably shorter than the first internode in a normal 

 race, although the coleoptiles are the same length. Van Over- 

 beek (1935) has shown that the inhibited development of this 

 first internode is due to the destruction of auxin, the amount 

 produced being about the same in seedlings of nana and normal. 

 A given amount of growth hormone applied unilaterally to the 

 coleoptile stumps of normal and nana resulted in smaller curva- 

 ture of the latter (Fig. 34), thus supporting the conclusion regard- 

 ing destruction. This destruction is correlated with a greater 

 catalase and peroxidase activity; hence van Overbeek concluded 

 that the dwarf type of growth in this variety of maize must be 

 due to a more active oxidation system. By raising the tempera- 

 ture of normal seedlings to 60°C. for one hour, thus increasing the 

 catalase activity, it was possible to make the normal into dwarf. 



