GROWTH SUBSTANCES FOR NORMAL GROWTH 



89 



removed, Bremekamp (1925) and Dolk (1929a) showed that no 

 geo-growth reactions appear in the Avena coleoptile. Navez and 

 Robinson (19326) came to the same conclusion. 



Growth Substances and Normal Growth. — As has been mentioned 

 previously, Paal (1918) showed that growth substance is being 

 formed continuously in the nonilluminated coleoptile tip, whence 

 it migrates into the more proximal portions of the coleoptile and 

 promotes growth. 



Rothert (1894) and Stark (1917) showed that the removal of 

 the coleoptile tip produces a retardation of growth in the coleop- 

 tile stump, a fact that Soding confirmed when he investigated this 

 same question (1924, 1925, 1929). The rate of growth (Table 3) 



Table 3. — Growth ix Length of Normal and Decapitated Avena 



coleoptiles 

 The figures in the table are average values from Tables I to III of Soding, 



1925 (p. 589) 



Treatment 



Increase 



in the first 



5 hr. 



Increase 

 in the fol- 

 lowing 13 

 hr. 



A. Decapitated 



B. Decapitated, the tip replaced, and again' re- 



moved after 5 hr 



C. Intact control plants 



2.57 



1.65 

 3.40 



in the first 5 hours after decapitation was only 42 per cent of that 

 in normal seedlings. Furthermore, the rate of growth of the 

 coleoptile stump was increased about 49 per cent in the first 

 5 hours when the removed tip was again replaced. Soding's 

 experiments showed that the rate of growth of normal plants is 

 not reached in decapitated plants in the first few hours, even with 

 their tips replaced, probably because the transport of growth 

 substance is retarded by the wound. After 10 to 14 hours, even 

 ^^ithout replacing the tip, the rate of growth of decapitated 

 coleoptiles became about the same as that of normal seedlings. 

 This increase in growth was brought about by ''physiological 

 regeneration" of the tip, which produced about the same amount 

 of growth substance as the normal. 



It is clear from this that a substance is dispersed from the tip 

 which promotes growth in the basal region. If, instead of replac- 



