REGULATORY SUBSTANCES 



511 



organisms) involve the production of growth-provoking or wound- 

 heahng hormones. 



The most recent work on plant-growth hormones is that of 

 Kogl, Thimann, and Went. Previous to them, Paal had shown 

 that the tip of a growing root contains a growth-stimulating sub- 

 stance; if the tip is removed, growth ceases. Injury is not 

 responsible, for if the severed root tip is replaced, growth con- 

 tinues ; but if a thin sheet of tinfoil or mica is placed between tip 

 and stump, no growth results. The same tip is not necessary; 

 one from another root will do as well, but a piece out of the base 



Fig. 178. — Extraction of the growth hormone from root tips placed on agar 

 blocks (lower left). Continued vertical growth (lower center) or curved growth 

 (lower right) results when the blocks are substituted for the tips. (From F. 

 Went.) 



of the root will not do. If the tip is replaced off center, growth is 

 greater on the one side than on the other, and the root curves. 

 Went was able to extract the growth hormone by placing root 

 tips on agar, gelatin, or silica gel and then putting the gel blocks 

 in the place of the root tips; growth continued vertically when the 

 block was centrally placed, and laterally, forming a curve, when 

 the block was placed off center (Fig. 178). The growth-stimu- 

 lating hormone was isolated by Kogl and named auxin; it con- 

 sists of three forms — auxin a, C18H32O5; auxin 6, C18H30O4; and 

 heteroauxin, C10H9O2N. The amount in a root tip is extremely 

 small, but it has now been obtained in larger quantities from 



