7O LECTURE V. 



cellulose walls, and, in addition to protoplasm and cell-sap, 

 they contain numerous chlorophyll-corpuscles : a full account 



FIG. 13. Transverse section of the lamina of a leaf: a, the cuticle; b, the epi- 

 dermis of the upper surface ; c, the pallisade-parenchyma ; d, the spongy 

 parenchyma ; c and d together constitute the mesophyll j <?, epidermis of lower 

 surface ; f, intercellular spaces ; g, guard-cells of a stoma. The cells of the 

 mesophyll contain chlorophyll-corpuscles. 



of the structure of these bodies will be given hereafter. The 

 epidermal cells usually contain no chlorophyll-corpuscles, 

 and their external walls are much thickened and cuticu- 

 larised : moreover they are packed closely together, so that 

 they form a membrane, the continuity of which is inter- 

 rupted here and there by intercellular spaces, called stomata, 

 each of which is bounded by usually two specially modified 

 epidermal cells, the guard-cells. The stomata open internally 

 into the intercellular spaces of the mesophyll, and thus the 

 air can have free access to the interior of the leaf, and from 

 this to the rest of the plant. That the stomata do thus 

 communicate with the intercellular spaces can be proved 



