=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 centre 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_. [Illustration: FIG. 72.—PART OF CROSS-SECTION OF ROOTSTOCK OF ASPARAGUS, 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 ring._ As the dicotyledonous seed germinates, five bundles are usually formed in its hypocotyl (Fig. 74); soon five more are interposed between them, and the multiplication continues, in tough plants, until the bundles touch (Fig. 74, right). The inner parts thus form a ring of wood and the outer parts form the inner bark or bast. A new ring of wood or bast is formed on stems of dicotyledons each year, and the age of a cut stem is easily determined. [Illustration: FIG. 73.—THE SCATTERED BUNDLES OR STRANDS, in monocotyledons at _a_, and the bundles in a circle in dicotyledons at _b_.] [Illustration: FIG. 74.—DICOTYLEDONOUS STEM OF ONE YEAR AT LEFT WITH FIVE BUNDLES, and a two-year stem at right. _o_, the pith; _c_, the wood part; _b_, the bast part; _a_, one year’s growth.] [Illustration: FIG. 75.—FIBRO-VASCULAR BUNDLE OF INDIAN CORN, much magnified. _A_, annular vessel; _A′_, annular or spiral vessel; _TT′_, thick-walled vessels; _W_, tracheids or woody tissue; _F_, sheath of fibrous tissue surrounding the bundle; _FT_, fundamental tissue or pith; _S_, sieve tissue; _P_, sieve plate; _C_, companion cell; _I_, intercellular space, formed by tearing down of adjacent cells; _W′_, wood parenchyma.] [Illustration: FIG. 76.—THE DICOTYLEDONOUS BUNDLE OR WOOD STRAND. Upper figure is of moonseed: _c_, cambium; _d_, ducts; 1, end of first year’s growth; 2, end of second year’s growth; bast part at left and wood part at right. Lower figure (from Wettstein) is sunflower: _h_, wood-cells; _g_, vessels; _c_, cambium; _p_, fundamental tissue or parenchyma; _b_, bast; _bp_, bast parenchyma; _s_, sieve-tubes.]