84 



GROWTH HORMONES IN PLANTS 



-COTYLEDON 



COT/LEDON 

 I 



PORE- 



I 



C D 



Fig. 23. — Median longitudinal sections of Avena coleoptiles at different stages of develop- 

 ment. A and B, diagrams through germinating embryos, X 11. C and D, detailed draw- 

 ings of coleoptiles, 1.62 mm. and 4 mm. in length, respectively, X 40. Mitoses are evident 

 throughout. From D to maturity, the 1 mm. segments a, b, c. and d elongate to the dimen- 

 sions indicated by the diagram in Fig. 24. The heavy line beside C and D indicates the 

 relative length of the coleoptile, to be compared with the heavy Mne in Fig. 24. (After 

 Avery and Burkholder, 1936.) 



