82 



PHYTOHORMONES 



two factors must be relatively fast as compared with their 

 rate of combination, and hence neither is limiting over a 

 considerable zone. Avena grown in red light is intermediate 

 between these two (Figure 35, b). Another variation of 



a: 



Fig. 35. Diagrams of types of growth in seedlings. Ordinates, relative 

 growth rates; abscissae, zones of the shoot. Relative amount, and direction 

 of movement, of auxin, A, and food factor, F, mdicated by length and direc- 

 tion of arrows; their rate of combination indicated by rapid, medium, or slow. 



this growth type is that in which the growth is almost 

 entirely apical, as in Pisum seedlings (Figure 35, d). This 

 is comparable with the Zea mesocotyl, but the growing 

 zone is shifted to the tip, doubtless on account of the very 

 low auxin supply, only small amounts of auxin being obtain- 

 able from apical buds of Pisum by diffusion (u). 



The growth of the mesocotyl of grasses merits special 



