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(also cork), the secondary integumentary tissue that forms the external part of the periderm. The phellem originates from cells of the cork cambium (phellogen) that have divided tangentially.
In woody plants, phellem forms on trunks, branches, roots, and bud scales; sometimes it develops on tubers and fruits (muskmelon and pear). In herbaceous dicotyledons, phellem usually covers the roots and hypocotyl; among monocotyledons it characterizes some palms (coconut), dracenas, and century plants. Phellem also forms at sites of injury.
Phellem cells are dead. As a result of suberization, their membranes are impermeable to liquids and gases, and their cavities are filled with air and resinous substances. Phellem protects the plant from excessive evaporation, temperature fluctuations, penetration by microorganisms, and ingestion by animals.
Soft phellem, consisting of thin-walled cells, characterizes chokeberry and fir; hard phellem, with cells having thick walls, is typical of willow. In birch bark, layers of cells with hard walls and brown contents alternate with layers of thin-walled cells filled with betulinol, a white resinous substance. The phellem of the tulip tree, spindle tree, pine, and larch consists of thin-walled suberized cells and phelloids. The latter are cells with thick walls that are stratified, porous, and lignified but not suberized (sclerotic phellem). Phellem forms ribs or winglike processes on the young trunks and branches of the smooth-leaved elm, spindle tree, and common maple.
The voluminous, annually increasing phellem of the cork oak is used principally to hermetically seal bottles of fine wines, juice, and mineral water. Cork is also used in the manufacture of linoleum, insulation, packing, floats, and life buoys.
REFERENCESRazdorskii, V. F. Anatomiia rastenii. Moscow, 1949.
Iatsenko-Khmelevskii, A. A. Kratkii kurs anatomii rastenii. Moscow, 1961.
Esau, K. Anatomiia rastenii. Moscow, 1969. (Translated from English.)
L. I. LOTOVA