o, the pith; c, the wood part; b, the bast part; a, one year’s growth.
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.
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.
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 of a lighter colour 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.
Fig. 77.—White Pine Stem, 5 years old. The outermost layer is bark.