Q.Describe the internal structure of a dicotyledonous stem in transverse section, and explain the process of secondary growth by which such a stem increases in girth.
Primary structure. A transverse section of a young dicot stem shows, from the outside in: a single-layered, cuticle-covered epidermis, bearing occasional stomata and trichomes; a hypodermis of a few layers of collenchyma just beneath it, giving the still-elongating stem mechanical support without sacrificing flexibility; a parenchymatous general cortex, whose innermost layer is often visibly starch-storing; a discontinuous ring of sclerenchyma patches called bundle caps marking the pericycle, sitting directly above each vascular bundle; and, arranged in a single ring around a large, central, parenchymatous pith, the vascular bundles themselves — each conjoint, collateral and open, with a strip of cambium lying between an inner xylem and an outer phloem. Narrow parenchymatous medullary rays run radially between adjacent bundles, connecting pith to cortex.
Secondary growth. The cambium strip already present within each bundle (fascicular cambium) is joined by newly meristematic parenchyma in the medullary rays (interfascicular cambium), so the two together form one continuous ring of vascular cambium running around the whole stem. This ring divides repeatedly: toward the inside it cuts off cells that mature into secondary xylem, and toward the outside it cuts off cells that mature into secondary phloem. Because far more secondary xylem accumulates than secondary phloem over time, the bulk of an older stem's cross-section becomes wood. Where the growing season varies through the year, the cambium alternates between producing wide-lumened, less dense spring wood and narrow-lumened, denser autumn wood; one band of each together forms a single annual ring, letting the stem's age be estimated by counting rings.
At the same time, a second lateral meristem, the cork cambium (phellogen), typically arises within the outer cortex once the original epidermis can no longer stretch to cover the thickening stem. It cuts off suberised, dead cork (phellem) to the outside for protection, and a small amount of living phelloderm to the inside; the phellogen, phellem and phelloderm together form the periderm, interrupted by small pores called lenticels that keep the inner tissues in gas exchange with the atmosphere despite the otherwise impermeable cork layer.
Primary structure: epidermis → collenchymatous hypodermis → cortex → sclerenchymatous pericycle patches → ring of open, collateral bundles around a large pith. Secondary growth: vascular cambium (fascicular + interfascicular) adds secondary xylem/phloem, producing seasonal spring/autumn wood (= annual rings); cork cambium adds the periderm (cork + lenticels), replacing the epidermis.
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