Plant organs are made up of cells. The most basic cell type, which makes up the ground tissue in plants, is the parenchyma cell (FIG. 7.3). Although all tissue types contain parenchyma, certain tissues are predominantly parenchyma, including the cortex and pith in stems and roots, and the mesophyll in leaves. Parenchyma cells are alive at maturity, have primary walls that are relatively thin, and can vary in their shape, from elaborately branched to almost isodiametric. Because they contain the full complement of cellular organelles, parenchyma cells have the potential to become meristematic and are totipotent, that is, they contain all the genetic material to develop an entire plant. They are a general-purpose cell and function in photosynthesis, so they may contain chloroplasts, and in storage of water, photosynthates (reserve foods), and many other compounds. Because of their thin walls (FIG. 7.3) and usually high water content, parenchymatous tissue is not generally an important structural component of plants, except in some of the earliest land plants (see Chapter 8). Parenchyma cells are less commonly preserved in fossils than some other cell types, especially sclerenchyma.
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