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 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.

, in monocotyledons at a, and the bundles in a circle in dicotyledons at b.

, and a Two-year Stem at Right.

o, the pith; c, the wood part; b, the bast part; a, one year's growth.

, 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; l, intercellular space, formed by tearing down of adjacent cells; W', wood parenchyma.

When cross-sections of monocotyledonous and dicotyledonous bundles are examined under the microscope, it is readily seen