24 



PHYTOHORMONES 



B. Evolution of the Avena Test Method 



By placing the cut tip asymmetrically on the stump, 

 Paal (1919) showed that the growth of the lower zones is 

 accelerated by a diffusible substance coming from the tip. 

 His experiments were carried out with seedlings of Coix, 

 and were afterwards confirmed on Avena by Nielsen (1924). 

 This conclusion was confirmed by the careful measurements 

 of growth made by Soding (1923, 1925). He compared first 

 the growth of the stump, after decapitation, with the growth 

 of the corresponding portion of the intact coleoptile. In 

 the first 5 hours the growth was only about 40 per cent of 

 that of the intact control. If, however, the tip were replaced, 

 the growth of the stump was accelerated to 60-70 per cent 

 of that of the intact control, thus proving that the decrease 

 in growth after decapitation was not primarily due to any 

 effect of the wound, such as the "wound substances" of 

 Stark (1921). On the other hand the accelerating effect 

 of the tip must be due to its secretion of growth-promoting 

 substances. Soding states: "Since these substances cer- 

 tainly also influence growth in the intact plant, I propose 

 to call them growth hormones." Cholodny (1924) carried 

 out a similar experiment on Mais coleoptiles, in which the 

 growth of coleoptile stumps on which the tips had been 

 replaced was, in 3-4 hours, 144 per cent of that of the un- 

 treated stumps. 



When the growth measurements were continued beyond 

 the first 5 hours, however, a new phenomenon appeared. 

 In a representative experiment Soding obtained the follow- 

 ing average growths in mm. (Table II). 



TABLE II 



