GROWTH SUBSTANCES FOR NORMAL GROWTH 85 



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Fig. 24. — Detailed drawings of cells in longitudinal sections from different levels of a 

 mature Avena coleoptile. The heavy line at the left indicates the relative length of the 

 marked segments designated a, b, c, and d in proportion to the over-all length of the coleop- 

 tile. Compare with Fig. 23. The drawings represent part of the apical end of segment a 

 and small portions from the middle of segments b, c, and d. Compare the small size of the 

 cells near the apex with the elongated cells in the lower portions. Note the extreme length 

 of the epidermal cells. {Adapted after Avery and Burkholder, 1936.) 



