94 



GROWTH HORMONES IN PLANTS 



20.4 



-36.9 



72 



20.4 



33.6 



Fig. 30. — Diagrams of Nicotiana leaves showing growth-hormone content 

 (expressed in plant units) at different ages and in different portions. A, young 

 leaf. C and D, older leaves from plants grown in a greenhouse. B, leaf of same 

 age as C, but kept in dark for 10 days, followed by 1 day in the light. The auxin 

 concentration gradient shown in A and C is due to accumulation in the midrib 

 and movement toward the base of the leaf. In contrast, leaf B shows less 

 accumulation at the base (data in parentheses) . Note disappearance of growth 

 hormone at the distal end of the older leaf, D. {After Avery, 1935.) 



= RELATIVE GROWTH 

 INTENSITY 



= POLARIZED GROWTH 



A B 



Fig. 31. — Growth of the Nicotiana leaf. A, showing greatest growth intensity 

 {localized growth) in marginal and basal regions, as indicated by the density of 

 stippling. B, the segments indicated in the distal and proximal portions of the 

 leaf show a relatively greater increase in length than in width. While this 

 polarized growth is not pronounced at the apex toward the end of the growth 

 period, it is very striking at the basal end of the leaf, where it is correlated with 

 higher concentrations of growth hormone. {After Avery, 1935.) 



