MULSOW : DIFFERENTIATION OF OAKS. 273 



mm. thick in Q. coccinea, .02 mm. in Q. schneckii, and about 

 .016 mm. in Q. macrocarpa. The cell walls of Q. rubra are also 

 slightly thicker than in any of the others. (For the compari- 

 son, see figs. 13, 14, 15, 16.) 



Leaf Parenchyma. 

 The palisade cells of Q. macrocarpa are slightly more numer- 

 ous than in any of the others, there being 22,000 per sq. mm. in 

 Q. macrocarpa, 21,100 per sq. mm. in Q. rubra, 20,200 per sq. 

 mm. in Q. schneckii, and 16,600 per sq. mm. in Q. coccinea. 

 (See figs. 5, 6, 7, 8.) The cross section of the leaf shows the 

 palisade cells of Q. schneckii to be shorter than the others, 

 being .035 mm. long, while in the other leaves they are about 

 .045 mm. in length. This seems to account for the thinness of 

 the leaf of Q. schneckii. (See figs. 13, 14, 15 and 16.) 



Lower Epidermis. 



The sizes of the cells in the lower epidermis are nearly the 

 same in all except in Q. macrocarpa, there being 10,960 cells 

 per sq. mm. in Q. macrocarpa, and in the others they run thus 

 from 4920 in Q. schneckii to 4900 in Q. rubra and 4870 per sq. 

 mm. in Q. coccinea. The thicknesses of the lower epidermis are 

 various in the different species. In this respect any measure- 

 ments give Q. rubra .016 mm., Q. schneckii .015 mm., Q. 

 coccinea .012 mm., and Q. macrocarpa .010 mm. The lower 

 epidermis of Q. macrocarpa also has persistent hairs in tufts, 

 while in the others the hairs fall off when the leaves are quite 

 young. 



Stomata. 



The stomata are found on the lower side of the leaf. They 

 vary among the different species as to number, size and shape. 

 The number of stomata per square millimeter was found by 

 taking the average of twenty-five fields from various parts of 

 the leaf, except where a vein occupied a whole field such a 

 field was not counted. The fields counted were .09 sq. mm. in 

 area. The number of stomata per square millimeter was 

 found to be 375 in Q. schneckii, 500 in Q. coccinea, 515 in Q. 

 rubra, and 819 in Q. macrocarpa. The stomata of Q. schneckii 

 are of two sizes, the larger size being the more numerous and 

 of elliptical shape, while the smaller size are almost round. 

 The stomata of Q. rubra and Q. coccinea are almost round, but 

 in Q. macrocarpa the stomata are quite elliptical and smaller 



