bunton: lesquerella spathulata. 203 



the conduction of water. Figure 63 represents a small portion 

 of the midrib as seen in longitudinal section: A, spiral tra- 

 cheary tracheid; B, undivided mother cell of the sieve tubes. 



The Photosynthetic Tissues. — The leaf of Draba is bifacial, 

 that is, palisade cells occur on both sides of the leaf, which is 

 from twelve to fifteen layers of cells thick. The average radial 

 length of a palisade cell is 0.027 mm., the average cross diame- 

 ter 0.02 mm. Figure 64 represents a few palisade cells as 

 seen in surface view. 



The average number of chloroplasts in a single mesophyll 

 cell on either side of the leaf is twenty-seven. The small num- 

 ber of chloroplasts found in the mesophyll cells of the upper 

 side of the leaf allows the sunlight to penetrate to the lower 

 side, thus giving to the cells there the power to carry on photo- 

 synthesis effectively. 



The Water-storage Tissues. — In the leaf of Lesquer'ella there 

 are no water-storage cells as are found in that of Townsendia. 

 The only structures in the leaf that might serve principally 

 as water-storage elements are the large trichomes which occur 

 on both sides of the leaf. These trichomes, as shown in figure 

 65 (surface view) are quite frequent, and are stellate-peltate 

 in form. These, as a rule, are found above a vein, and, since 

 they are alive even on old leaves, they presumably have some 

 active function to perform. Their basal portion is slightly 

 depressed beneath the lower level of the epidermis, and the 

 wall here is cellulose. The remaining portion of the wall of 

 the trichomes is cellulose, except the very outer portion, which 

 is cutinized. The fact that these trichomes occur above the 

 veins and that their basal portion is cellulose-walled seems 

 to point to the conclusion that when there is plenty of water 

 furnished by the veins they absorb water, store it up, and 

 yield it to the mesophyll cells as it is needed. Figure 66 rep- 

 resents a trichome as seen in a leaf cross section : A, cutinized 

 portion of a wall of trichome; B, cellulose portion of wall of 

 trichome ; C, mesophyll cells ; D, tracheids. 



The Aerating Spaces of the Leaf. — Mounting the bleached 

 leaf so that it could be looked through from surface to surface, 

 I estimate that approximately 2.7 per cent of the volume of the 

 leaf is devoted to intercellular spaces ; in other words, the vol- 

 ume of the intercellular spaces and the volume of the whole 

 leaf, roughly estimated, stand in relation to each other as 



