90 GROWTH HORMONES IN PLANTS 



ing the tip after decapitation, one covers the wound with agar 

 containing growth substance, the rate of growth can be increased 

 far beyond the normal (Fig. 1) (Went). It has been found that 

 when the growth-substance content of the agar amounts to 

 100 WAE (Boysen Jensen, 1933a), the coleoptile stump surpasses 

 the enclosed leaf in growth, which normally never occurs. 

 Soding (1929) investigated different portions of the coleoptile for 

 growth substance and found that the amount decreased greatly 

 from tip to base. This observation has been confirmed by the 

 work of Thimann (1934). 



From these and other experiments, it has been concluded in the 

 past that growth substance is formed exclusively in the tip under 

 normal conditions and that it migrates from there into the more 

 proximal portions of the coleoptile where it stimulates growth. 

 In view of the upward movement of growth substance which has 

 been demonstrated in certain plants by Zimmerman and Wil- 

 coxon (1935), it appears equally probable that the hormone or its 

 precursor is being formed in the endosperm (Cholodny, 19356) 

 and moved upward in the vascular system to the tip, from which 

 point it is dispersed downward. In fact, Pohl (1935) concludes 

 from a series of important experiments that the coleoptile tip 

 does not produce growth substance but can only activate the 

 reserve stored in the endosperm. The phenomenon of "physi- 

 ological regeneration" apparently could be explained by this 

 interpretation. Further confirmatory evidence is found in the 

 observation that physiological regeneration (Soding, 1929) takes 

 place just as vigorously whether the coleoptile is decapitated at 

 the tip or several milhmeters below and Heyn (1935) has found 

 that physiological regeneration does not take place when the 

 coleoptile is separated from the food stored in the seed. 



The hypothesis that the decrease in rate of growth after 

 decapitation may be caused by lack of growth substance has 

 been disputed by Priestley (1926rf) and by Tetley and Priestley 

 (1927). When the coleoptile is decapitated, water exudes from 

 the cut surface; this loss of water is, according to Priestley, the 

 essential reason for the retardation of growth, and he contended 

 that retardation must persist until the supply of water is rendered 

 normal again by healing of the wound. The promotion of 

 growth by replacement of the tip was explained by partial closing 

 of the wound. It may be said here that Priestley's explanations 



