Page:The Oak.djvu/99

 formed in the cells of the leaves. The water, with salts in solution, coming from the soil after it has been absorbed by the root-hairs, passes up the wood (xylem) of the roots and stem, through the vessels of the petioles and leaf-venation, and is finally distributed to the cells of the mesophyll; the substances formed in these cells then pass down by the phloëm (sieve-tubes, etc.) of the venation and leaf-stalk, and thence are distributed to other parts of the plant.

Now let us look at the mesophyll which these vascular bundles support and serve as conduits for. It consists of two distinct parts (Fig. 22). Beneath the upper epidermis, the cells of which are fitted closely together without intercellular spaces and are devoid of chlorophyll corpuscles, there are one or two rows of vertical sausage-shaped cells, closely arranged like the wooden railings of a complete palisade—consequently they are termed the palisade cells. The lower moiety of the mesophyll, on the other hand, is composed of irregular cells with large intercellular spaces between them, and this loose, spongy tissue, as it is aptly called, abuts below on the lower epidermis. Both the palisade cells and those of the spongy tissue contain numerous chlorophyll corpuscles, as said.

This lower epidermis is worth a few minutes' consideration. It, like the upper epidermis, is also composed chiefly of closely fitting cells devoid of chlorophyll corpuscles, excepting that here and there we notice pairs of smaller cells containing chlorophyll—each pair with