cambium(redirected from cambia)
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the formative tissue, or meristem, primarily in the roots and stems of dicotyledonous and gymnospermous plants. The cambium is in the shape of a monostichous concentric cylinder (in cross section, it appears annular). As a result of cambial activity, the axial organs grow in thickness. Secondary phloem is produced toward the outside of the axis, and secondary xylem toward the inside. Cambium originates from the cells of the procambium, which lie between the primary phloem and primary xylem. The pericycle plays an important role in the formation of the cambium in roots. It appears that there is no cambium in leaves; however, if there is any, its activity terminates at an early stage in leaf development.
Cambial cells are prosenchymatous in form—elongated, pointed (with beveled ends), and flat. They extend the length of the plant. Cambial cell walls are soft and consist of cellulose; the primary pit fields contain plasmodesmata. Cambial cells divide, becoming phloem cells (toward the outside of the cambium) or xylem cells (toward the interior of the plant). Usually, much more xylem is formed than phloem. As a result of the division of some cambial cells, tiny cells are formed; these make up phloem-xylem rays. The phloem forms on one side of the ray; the xylem, on the other.
During the autumn and winter, cambial activity ceases (intemperate zones). The periodic activity of the cambium resultsin the formation of annual rings. Depending on the character ofcell division, cambium is divided into storied and nonstoriedtypes. In some dicotyledons (such as beets), additional cambiallayers form consecutive rings toward the periphery (polycam-bium formation).
O. N. CHISTIAKOVA