AUXIN AND GROWTH 



85 



hypocotyl. Dijkman, however, found that the degree to 

 which such hypocotyls will grow out after decapitation 

 depends on their initial length, which fits in better with 

 the above view of a flooding with auxin in the earlier stages 

 of growth. Production of auxin all over the hypocotyl 

 would be at variance with everything known for other 

 etiolated plants, and does not explain the experiments 

 below. Fliry (1932) found that the addition of auxin and 

 sugar, or replacement of the tip, gave better growth of 

 decapitated hypocotyls than auxin alone, which confirms 

 the above view that food factors are limiting in these seed- 

 lings (see Table VI). 



TABLE VI 



Growth Increments, in mm. per 20 Hours, of Decapitated Helianthus 

 Hypocotyls with Various Applications. After Fliry (1932). Mean op 

 10-14 Plants in Each Group 



In Type 3 of du Buy and Nuernbergk the growth is still 

 more restricted to the apex, as in etiolated Pisum sativum 

 stems (c/. Figure 35, d). Since the growing zone is so lim- 

 ited, tropistic curvatures do not travel very far down 

 the stem. This type is, in our view, quite unrelated to the 

 hypocotyl type (Figure 35, e) but is merely an extreme 

 case of a and b with low rate of auxin production. Corre- 

 spondingly little or no auxin can be obtained from such 

 stems by diffusion. 



Type 4 is exemplified by roots, the growth there also 

 being principally apical. As the function of auxin in the 

 growth of roots is not yet clear, they need not be considered 

 in this two-factor scheme. 



An early application of the two-factor scheme was made 

 by Dolk (1930) in his attempt to explain the migration of 



