secondary thickening


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Fig. 277 Secondary thickening . Secondary thickening in a typical dicotyledon.

secondary thickening

or

secondary growth

the growth in the girth of stems and roots in DICOTYLEDONS produced by division of the secondary MERISTEM, resulting in woody tissues. The growth begins by the formation of a continuous cambial ring. In stems there is already a fascicular cambium between the xylem and phloem of the vascular bundles which becomes joined up by interfascicular cambium. The xylem tissues become divided into an inner heartwood (dead) and an outer SAPWOOD.Secondary thickening can sometimes occur in herbaceous ANGIOSPERMS but is usually associated with woody types producing, for example, a stem which tapers in an upward direction. MONOCOTYLEDONS do not carry out secondary thickening. See Fig. 277 .In roots, a cambial ring is produced from parenchymatous cells between the XYLEM and PHLOEM.