Environmental Engineering Reference
In-Depth Information
The meristematic tissues and the production of new cells
following division from existing ones is a process that needs
to be elucidated, for it has implications regarding the types
of plants that should be used for phytoremediation of
contaminated groundwater. Meristem cells were first
recognized by Caspar Wolff in 1759 (Table 1.3). The meri-
stem is at the center of the stem, but the cells that divide
occur at the tip, or terminal buds above ground or root tips
below ground. These shoot and root meristems are charac-
teristic of most plants but are more important in annuals that
experience primary growth patterns. The meristems,
whether in the tips of shoots and roots or in the cambial
layer, are formed of undifferentiated cells that contain
mostly cytoplasm and a few organelles along with a small
vacuole, that later become differentiated into the various
cells of the plant, such as epidermis, xylem, phloem, etc.
These undifferentiated cells are the source of continued self-
renewal in plants (Weigel and J
Shoots grow in length from the terminal bud, where
active cambium is located. Smaller buds are located farther
down the stem and are called lateral buds. When the main tip
is removed, by natural damage or pruning, the food and
water that had gone to the removed tip is diverted to the
lower remaining buds, and they will grow at increased rates.
Anyone who has pruned a plant has experienced this phe-
nomenon. On the other hand, removal of too many leaves
after the buds have broken decreases potential food produc-
tion. Many lateral buds remain dormant for the life of the
plant. These dormant lateral buds combined with the fact
that all buds, both leaf and flower, are formed during the
previous growing season, help to ensure plant survival
across a range of environmental stresses over time.
Growth also is enhanced by the osmotic uptake of water
and resultant cellular swelling, or turgor. This stresses the
cells by inducing the cell wall to stretch, and growth occurs
by making this stretched cellular dimension a permanent part
of the plant. As this is an increase in water pressure or
potential, and water transport to leaves is along a gradient
of decreasing water potentials (as we will see later in this
chapter), a dilemma exists between water transport and
cellular turgor maintenance. For example, in the spring
when buds break, leaves will not enlarge and enable addi-
tional water transport unless water is initially available.
An increase in girth is accomplished by the cambium
stem cells that create new xylem toward the center and
new phloem toward the bark, with older xylem dying and
providing support, and the phloem dying and becoming the
cork and bark. Such cambial stem cells are characteristic of
woody plants that have primary and secondary growth
patterns. Less is known about these cambial stem cells than
stem cells at the tips of shoots and roots.
An interesting feature of many woody plants is the rela-
tively small proportion of the plant that grows compared to
its overall biomass. Typically, less than 1% of a woody plant
is, in fact, alive. This is because the nonliving parts of the
vascular system can support the plant without the costs of
metabolism. If this internal tissue rots, the tree will still
survive, until it falls from lack of support; this explains
why recently fallen trees were alive even though they had
been hollowed out by decay. An interesting mental image to
conjure is of a tree reduced to only its living cells; it would
appear as a thin column of green cells linked to leaves above
and white roots below, similar to the structure of a bubble,
with little surface area but of high volume. In fact, even
though plants are one of the longest living organisms on
earth, such as the aforementioned bristlecone pine that can
live to over 1,000 years of age, the actual growing cambium
is only a few years old; the rest of the tree is composed of
long-dead tissue. This is a consequence of the meristems, in
which the growing cambial cells are replaced continually as
other cells die.
urgens 2002). As such, these
meristem cells are analogous to human stem cells, which are
undifferentiated cells that have the potential to be used for
many purposes.
As the meristem cells divide, they move from the center
of the shoot or root tip to the tip and then back and down
around the original center. The meristematic cells divide
rapidly at first and then stop. Additional increase in size is
by cell-wall relaxation and elongation. In fact most increase
in plant size is actually an increase in the size of existing
cells rather than the addition of new cells. This is exactly
opposite of most animals, in which growth is a result of
cell division. After the elongation is over, the cell wall
re-hardens.
The consequence of the division of meristem cells is hard
to observe on a small scale, but large-scale consequences can
more readily be observed. One example is the leaves of
palms that are produced from the meristem at the tip of the
plants and grow outward and then downward as younger
leaves are generated. The unique bark of such plants is
composed of older leaf petioles, with the older ones near
the ground and progressively newer ones nearer the top. As
these undifferentiated meristem cells divide, some become
leaves where others become branches.
Growth also can be classified in plants as one of two main
growth types—determinate or indeterminate. Determinate
growth refers to a predictable growth cycle with the cessa-
tion of growth once a particular biomass is achieved,
followed by death. Plants that exhibit determinate growth
are typified by smaller plants, such as annuals. On the other
hand, plants that tend to keep on growing over time for many
years, even hundreds of years, exhibit indeterminate growth.
This is exemplified by woody plants, such as bristlecone
pines in the southwestern United States. In both cases, the
overall biomass achieved by either type of plant is controlled
by the genetic information contained in the cells.
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