GROWTH SUBSTANCES FOR NORMAL GROWTH 99 



i.e., to induce the destruction of auxin and obtain a short first 

 internode. 



Roots. — Roots are a continuation of the axial system of the 

 plant but differ sharply from the shoot in structure and behavior. 

 The region of primary growth in the root is so well-known that it 

 is unnecessary to describe it here. With regard to the presence 

 of growth substance in roots, there is considerable evidence 

 indicating that it is formed by the root tip; lesser amounts are 

 found proximally. 



Wiesner (1881) made the statement that the growth of roots in 

 contact with water is accelerated by decapitation. This observa- 

 tion was confirmed by Cholodny (1926); an increase in growth of 

 12 per cent took place after decapitation of the roots of Lupinus 

 angustifolius. Then Biinning (1928) investigated the effect of 

 decapitation upon the growth of the root; if the removed tip 

 portion w^as not too long, there was a temporary retardation of 

 growth in most roots, followed by an increase. Nielsen (19305) 

 also showed an increase in root growth as the result of 

 decapitation. 



To answer the question of how decapitation can produce an 

 increase in growth, Cholodny assumed that the growth substance 

 that is formed in root tips must be identical with that formed in 

 coleoptile tips. Even though the growth substance increases the 

 growth of the Avena coleoptile, it apparently retards the rate of 

 growth of the root. If this is so, the removal of the root tip 

 should bring about an increase in growth. Biinning (1927) con- 

 cluded, on the other hand, that the growth changes described 

 are to be construed as wound-growth reactions. 



It has been demonstrated repeatedly that growth substance 

 can influence the rate of growth of roots. Cholodny (1924) 

 showed that decapitated roots of Zea mays, upon which had been 

 placed the coleoptile tips of the same plant, grew 36 per cent less 

 rapidly than decapitated roots without tips. This indicates 

 that the growth substance of the coleoptile retards growth of the 

 root. This conclusion was confirmed by Nielsen (19306), who 

 determined the growth increase of the root when the 2 mm. por- 

 tion of the tip was first submerged in water and afterward in a 

 rhizopin solution. He showed that the rhizopin completely 

 inhibited the growth of roots of Lupinus alhus and Vicia Faha, 

 whether they were intact or decapitated. Moreover, the roots 



