Secondary Growth of StemsAn aquatic plant is buoyed by the water in which it grows, and its structural needs are simple. Land plants, however, require a structural support system. During the course of evolution when plants developed the ability to synthesize lignin—the polysaccharide that gives rigidity to the cell walls of wood—large, erect bodies were achievable, and their possessors became highly successful in colonizing the land. In modern plants, lignified wood cells are the secondary xylem cells. Most of the primary tissues outside of the vascular cambium are destroyed by the sideways push of the new cells, and a new group of secondary tissues—the bark—replace them.Formation of the secondary plant bodyDuring formation of the primary body, many plants retain meristematic tissues among differentiated ones. When stimulated to divide, these meristems, called cambia, produce new cells that, together
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