When cross-sections of monocotyledonous and dicotyledonous bundles are examined under the microscope, it is readily seen why dicotyledonous bundles form rings of wood and monocotyledonous cannot (Figs. 75 and 76). The dicotyledonous bundle (Fig. 76) has, running across it, a layer of brick-shaped cells called =cambium=, which cells are a specialized form of the parenchyma cells and retain the power of growing and multiplying. The bundles containing cambium are called _open bundles_. There is no cambium in monocotyledonous bundles (Fig. 75) and the bundles are called _closed bundles_. Monocotyledonous stems _soon cease to grow in diameter_. The stem of a palm tree is almost as large at the top as at the base. As dicotyledonous plants grow, the _stems become thicker each year_, for the delicate active cambium layer forms new cells from early spring until midsummer or autumn, adding to the wood within and to the bark without. As the growth in spring is very rapid, the first wood-cells formed are much larger than the last wood-cells formed by the slow growth of the late season, and the spring wood is less dense and lighter colored than the summer wood; hence the time between two years’ growth is readily made out (Figs. 77 and 78). Because of the rapid growth of the cambium in spring and its consequent soft walls and fluid contents, the bark of trees “peels” readily at that season. [Illustration: FIG. 77.--WHITE PINE STEM, 5 years old. The outermost layer is bark.] =Medullary Rays.=--The first year’s growth in dicotyledons forms a woody ring which almost incloses the pith, and this is left as a small cylinder which does not grow 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=. [Illustration: 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.