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 and strength to the plant, and of long tubular interrupted canals that serve to convey sap upward from the root and to convey food downward from the leaves to the stem and roots.

Monocotyledons, as shown by fossils, existed before dicotyledons appeared, and it is thought that the latter were developed from ancestors of the former. It will be interesting to trace the relationship in stem structure. It will first be necessary to learn something of the structure of the wood strand.

<, showing the wood (x) and bast (p) separated. Wood Strand in Monocotyledons and Dicotyledons.—Each wood strand (or fibro-vascular bundle) consists of two parts—the bast and the wood proper. The wood is on the side of the strand toward the center of the stem and contains large tubular canals that take the watery sap upward from the roots. The bast is on the side toward the bark and contains fine tubes through which diffuses the dense sap containing digested food from the leaves. In the root (Fig. 71) the bast and the wood are separate, so that there are two kinds of strands. , showing a few fibro-vascular bundles. An endogenous stem. In monocotyledons, as already said, the strands (or bundles) are usually scattered in the stem with no definite arrangement (Figs. 72, 73). In dicotyledons the strands, or bundles, are arranged in a