Fig. 71.—Diagram of Wood Strands or Fibro-vascular Bundles in a Root, showing the wood (x) and bast (p) separated.
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.
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.
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.
Fig. 73.—The Scattered Bundles or Strands, in monocotyledons at a, and the bundles in a circle in dicotyledons at b.
Fig. 74.—Dicotyledonous Stem of One Year at Left with Five Bundles, and a two-year stem at right.