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larger, even if the tree should live a century. It is not quite inclosed, however, for the narrow layers of soft cells separating the bundles remain between them (Fig. 78), forming radiating lines called medullary rays or pith rays.

Fig. 78.—Arrangement of Tissues in Two-year-old Stem of Moonseed.

p, pith; f, parenchyma. The fibro-vascular bundles, or wood strands, are very prominent, with thin medullary rays between.

The Several Plant Cells and their Functions.—In the wood there are some parenchyma cells that are still with thin walls, but have lost the power of division. They are now storage cells. There are also wood fibers which are thick-walled and rigid (h, Fig. 76), and serve to support the sap-canals or wood vessels (or tracheids) that are formed by the absorption of the end walls of upright rows of cells; the canals pass from the roots to the twigs and even to ribs of the leaves and serve to transport the root water. They are recognized (Fig. 79) by the peculiar thickening of the wall on the inner surface of the tubes, occurring in the form of spirals. Sometimes the whole wall is thickened except in spots called pits (g, Fig. 76). These thin spots (Fig. 80) allow the sap to pass to other cells or to neighboring vessels.

Fig. 79.—Markings in Cell Walls of Wood Fibers.

sp, spiral; an, annular; sc, scalariform.

Fig. 80.—Pits in the Cell Wall.

Longitudinal section of wall at b, showing pit borders at o, o.

The cambium, as we have seen, consists of cells whose function is growth. These