34 KANSAS UNIVERSITY SCIENCE BULLETIN. 



chamber (fig - . 19, a). The epidermal cells are rich in protein, 

 and in the young leaf they contain a small amount of starch. 



The stomata are of the usual type (fig. 18) . They are more 

 numerous on the upper surface and over the areas of the 

 chloroplast-bearing mesophyll (fig. 15), there being on the 

 average 126 to the sq. mm. on the upper surface, and 97 to 

 the sq. mm. en the lower surface. The guard cells are com- 

 paratively small, and are slightly raised above the surface 

 of the epidermis (fig. 19). Such a position may be of physio- 

 logical importance, since it increases the depth of the stomatal 

 opening. Brown and Escombe (2) have shown by their ex- 

 periments that whenever there is a constant difference main- 

 tained between the partial pressure of the vapor inside and 

 outside the leaf, the amount of diffusion of water vapor is 

 controlled by the depth of the apertures, and not by their 

 areas, the rate of diffusion varying inversely with the depth 

 of the stomatal opening. Thus the increased depth of the 

 stomatal tube would decrease diffusion, which decrease may, 

 however, be partly offset by the vapor density shells being in 

 such a position that they are readily removed by wind. 



Resin glands occur on the surface of the leaf, on the aver- 

 age 12 to the sq. mm. on the upper surface, and 3.4 to the 

 sq. mm. on the lower surface. (Fig. 12.) They increase in 

 number from the base of the leaf to its apex, and occur in clus- 

 ters, often as many as thirty in a group at the apex, and near 

 the tip of each tooth on the margin. (Fig. 14.) On the surface 

 of the leaf the glands are usually sunken to a depth of about 

 one-fourth the thickness of the leaf, and are always found in 

 connection with the water-storage cells which surround the 

 veins and extend to the epidermis. The layer of cells beneath 

 the glands are usually elongated, and have comparatively thin 

 walls (fig. 11, it) without a cuticle layer. 



In cross section the glands are round, with an average diam- 

 eter of .045, and are made up of from 20 to 60 cells. (Fig. 

 12, a.) A longitudinal section of a gland, however, presents 

 three to four tiers of cells, with the outer layer very regular 

 in form and arrangement. (Fig. 13, b.) The cells are rich in 

 cell contents, and have prominent nuclei. On the surface of the 

 leaf each gland has a thick coating of gum-resin, in the form of 

 a minute droplet, while on the margin the entire cluster is cov- 

 ered by the same gummy excretion. On the upper surface, 9.4 



