126 PLANT PHYSIOLOGY 



placed in the center of such a hollow stem, growth of the stem 

 will be considerably accelerated under the influence of the growth 

 hormone excreted by this tip. 



These experiments show that the growth hormone is not 

 specific; the excretion of the tip of the corn coleoptile increases 

 the growth rate of stems of the lupine and sunflower, and the 

 head of the dandelion can accelerate the growth of the peduncle 

 of the poppy or the coleoptile of oats. The growth-promoting 

 substance is capable of moving only in the direction from the 

 morphological top to the base, independent of the orientation 

 of the organ in space. According to Went, the mechanism 

 of its activity consists in the softening of the cell walls, which 

 facilitates their expansion. This hormone is utilized in the 

 process of growth, and the lower-lying zones are provided with 

 smaller quantities than those situated higher. This explains 

 the distribution of growth in the elongating organ, the grand 

 period of growth that has been described in the foregoing article. 



It is interesting to note that the root tip excretes a substance 

 that does not accelerate but inhibits growth of the root zone. 

 This is dependent not upon the properties of the substance but 

 on the peculiarities of the cells of the zone of elongation. Experi- 

 ments of Cholodny (1927) have shown that cut tips of roots 

 accelerate the growth of decapitated stem bases, and, conversely, 

 that tips of stems inhibit the growth of decapitated roots. Thus 

 the growth hormone proves to be identical, but various organs 

 react differently to the influence of this substance. 



The presence of the growth hormone may be revealed not only 

 in the tops of plant organs but likewise in other parts of the plant 

 and in secretions of the animal organism, e.g., in the saliva. But 

 the disco\'ery of this hormone in the urine of man and animals 

 was of special significance, for it permitted the Dutch chemist 

 Kogl to obtain it in quantities sufficient for detailed chemical 

 study. In its preparation, he made use of its solubility in ether 

 and insolubility in benzene, its capacity of giving insoluble salts 

 with lead and soluble salts with calcium, etc. After a series 

 of complicated manipulations, Kogl finally obtained 250 mg. of 

 a completely pure crystalline hormone, which he designated 

 ''auxin," and established its chemical nature. It proved to be 

 a monobasic acid of the empirical formula, C18H32O5, easily 

 transforming, when stored, into an isomeric completely inactive 



