92 



GROWTH HORMONES IN PLANTS 



An interesting technique was developed by Laibach and 

 Kornmann (1933a) to demonstrate the accelerating effect of 

 growth substance (extracted from pollen) upon growth in length 

 of the decapitated Avena coleoptile (Fig. 28). 



Went (1928a) suggested reasons for the distribution of growth 

 in the Avena coleoptile, stating that the rate of growth in the 



10 







_L 



_L 







14 



16 



18 



20 



22 



2 4 6 8 10 12 



Time in hours 

 Fig. 29. — Growth rate of the upper 10 to 15 mm. zones of Avena eoleoptiles 

 with different amounts of growth hormone, a, the normal course of growth; b, 

 growth of a group of eoleoptiles to which auxin paste was added at an early 

 stage; c and d, other groups of eoleoptiles to which auxin was applied at later 

 stages in growth. {After Went, 1935c.) 



basal portion is limited by the failing supply of growth substance ; 

 on the other hand, growth in the tip is limited by the lack of 

 organic material (supplied by the seed) which is necessary for cell 

 elongation. The rate of growth reaches a maximum at that 

 point where both food and growth substance are present in 

 sufficient amount, and the water supply is adequate. DuBuy 

 showed (1933) that growth in the coleoptile is gradually retarded 

 when the endosperm is removed ; aging is also mentioned as one 



