Page:The Oak.djvu/71

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53
THE SEEDLING AND YOUNG PLANT.

ent it must suffice to notice that the cells of this cambium cylinder go on developing into new xylem, or phloëm, or medullary rays, according to position and circumstances; meanwhile we are only concerned with the vascular bundles of the young shoot.

On the transverse section through the very young shoot, provided the preparation is thin and examined with a high power of the microscope, the young vascular bundles are found to present a definite and symmetrical structure, easily distinguished from that of the fundamental cell-tissue in which they are, so to speak, imbedded (Fig. 12).

The cells of the medullary rays are seen in one, two, or several rows, each cell having the form of a parallelopiped or ordinary brick—the bricks being supposed standing on their narrow sides and with the long axes directed radially. The walls in contact with the vascular bundles are thickened, and soon become woody and beset with simple pits; the cells contain protoplasm and nuclei, and in winter become filled to crowding with starch grains. They also contain tannin.

The young vascular bundles, in section, project into the pith—like wedges with a rounded point—giving to the latter the five-rayed shape on the transverse section already referred to (Fig. 9).

The cells of the pith also have their walls thickened and pitted, and also contain protoplasm, nuclei, and tannin, and starch in winter. At the rounded angles of the vascular wedges the cells are smaller than else-  5