PLATE IX. 



Fig. 16. Stoma of Q. muhlenbergii (semidiagrammatical) . Dotted line 

 shows position of inner cell wall when cell is turgid, (p', r'.) 



Fig. 17. Longitudinal section of guard cell along. line a-b, fig. 16. 



Fig. 18. Cross section of stoma along line m-n, fig. 16. r-r' shows 

 movement in opening. 



Fig. 19. Cross section of stoma along line x-y, fig. 16. p-p' shows 

 movement due to turgidity. 



Fig. 20. Lower epidermis of leaf of Q. rubra about 2 days old, show- 

 ing type and frequency of stomata. (Heavy lines.) X 405. 



Fig. 21. Lower epidermis of leaf of Q. rubra about 7 days old, show- 

 ing development of stomata. Same area as fig. 20. X 405. 



Fig. 22. Lower epidermis of leaf of Q. rubra at maturity, showing 

 type and frequency of stomata. Same area as figs. 20 and 21. x 405. 



Fig. 23. Frequency of stomata on leaves of No. 1. 1 sq. mm. X 67. 

 Black square in lower corner is 1 sq. mm. 



Fig. 24. Frequency of stomata on leaves of No. 2. 1 sq. mm. of leaf 

 surface. X 67. 



Fig. 25. Frequency of stomata on 1 sq. mm. of leaf surface of No. 3. 

 X 67. 



