86 



GROWTH HORMONES IN PLANTS 



200 

 180 

 160 

 140 



V) 



-! 120 



u 



L. 100 



o 



S 80 

 Z 60 



40 



20 



SUBEPIDERMIS 

 THIRD LAYER 

 FOURTH LAYER 



INNER EPIDERMIS 



-* OUTER EPIDERMIS 



10 



20 



30 



40 



50 



COLEOPTILE LENGTH IN MM, 



Fig. 25. — Graph showing the number of cells from tip to base in the different 

 layers of the Avena eoleoptile, at six stages in its growth. The outer epidermal 

 cells do not increase in number, while the cells of the other layers multiply 

 rapidly in the early period of development and then remain about constant in 

 number. A definite gradient of cell-division intensity, decreasing inward from 

 the subepidermis to the inner epidermis, is apparent for the first quarter of the 

 growth period. During the last three quarters of its growth period, the increase 

 in length of the eoleoptile is proportional to the elongation of its constituent cells. 

 {From Avery and Burkholder, 1936.) 



''{\' 



-A- - 



J ) 



4 mm. 

 Stage 



12 mm. 

 Stage 



20 mm. 

 Stage 



29 mm. 37 mm. 



Stage Stage 



(at maturity) 



Fig. 26. — Diagrams of the Avena eoleoptile to show shift in position of the 

 zone of maximum growth intensity at different stages of development. Coleop- 

 tiles 4 ram. in length were marked into four 1 mm. segments (a, b, c, d) and the 

 length of each was measured in several later stages of growth. The density 

 of the dots indicates relative growth intensity. Note that the region of maxi- 

 mum growth shifts from the base in the young eoleoptile (12 mm. stage) to 

 the apical region in a maturing eoleoptile. (Adapted after Avery and Burkholder, 

 1936.) 



