Page:The Oak.djvu/50

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32
THE OAK.

phloëm, and we may shortly examine the elements of which they are composed, as before, by comparing sections of various kinds.

Here, again, we find the chief structures in the phloëm are also vessels—i.e., long, tubular organs—but very different in detail from the vessels of the xylem.

In the first place, their walls are thin and soft, and composed of the unaltered cellulose which is so characteristic of young cells (instead of being hard, like the lignified wails of the xylem vessels); then, again, they contain protoplasm and other organized cell contents, instead of merely air and water. Finally, they are not so completely tubular as the typical xylem vessels are, because the transverse septa of the constituent cells are not absorbed, but are merely pierced by fine strands of protoplasm, and therefore look like sieves when viewed from above—whence the name "sieve-tubes." In the phloëm also we find cells—phloëm-cells—packed in between the sieve-tubes.

If we shortly summarize the above we find that the root consists of an axis-cylinder surrounded by a cortex and the piliferous layer. At the tip the whole is covered by the root-cap, which is organically connected with the embryonic tissue which forms all these structures. The axis-cylinder is somewhat complex; it is sheathed by the endodermis and the pericyle, the latter of which gives origin to the new rootlets. Inside the pericycle are the vascular bundles running up and down as separate, alternate cords of xylem and phloëm;