BSC 314 Lecture Notes - Lecture 12: Dendrochronology, Wood, Vascular Cambium
26 Jun 2018
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Secondary Growth of Stems
An 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 body
During 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 with the remaining primary tissues, form the secondary (woody)
plant body.
Vascular cambium
The vascular cambium lies between the primary xylem and phloem. It consists,
accurately, of only one layer of cells, but the first cells it produces cannot be
distinguished from cambial cells so the narrow area is sometimes referred to as the
“cambium” or the “cambial zone.”
Two kinds of meristematic cells, called initials, are recognizable in the cambium:
fusiform and ray initials. The fusiform initials are elongated vertically in the stem and
have tapering ends. They divide to produce the conducting cells of both the xylem and
the phloem (xylem toward the inside of the stem, phloem toward the outside).
Considerably more xylem cells than phloem cells always are produced. The ray initials
are smaller, more cuboidal and produce parenchyma in rows radiating out from the
center of the stem. The bands of parenchyma, called rays (vascular rays), conduct
water and dissolved materials laterally in the stem.
Wood: Secondary xylem
The structure of wood varies from species to species and between major groups. A
common categorization separates the softwoods of gymnosperms from
the hardwoods produced by angiosperms. (These are not very good descriptive terms
because of the great variability in density among species in both groups, but the groups
do differ in the kinds of cells in their wood.)
Gymnosperm wood. Softwood lacks vessels and is composed almost entirely of
tracheids. The rays are ribbon like structures of parenchyma, one cell wide and only a ‐
few cells deep. Vertical resin ducts or canals are characteristic of gymnosperms. The
ducts intercellular spaces lined with parenchyma tissue, the cells of which secrete resin
into the cavity in response to wounding.
Angiosperm wood. Hardwoods are harder than most softwoods because of the
numerous fibers present. The usual conducting cells (tracheids and vessel segments),
scattered parenchyma, and ray parenchyma are present in the wood. Some dicot
(eudicot) species have resin and resin ducts, but other substances—latex (rubber), for
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Document Summary
An 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. During 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 with the remaining primary tissues, form the secondary (woody) plant body. The vascular cambium lies between the primary xylem and phloem.
