Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 85

species that grow in swamps or other areas inundated by
standing water and anoxic conditions maintain an oxic halo
around their roots in the rhizosphere by transporting oxygen
from the lenticels to the roots (Hook et al. 1970).
Lenticels are necessary in order to supply the living cells
with oxygen to supply respiration of previously fixed CO 2 .
This is important during times of no light or for parts of the
plant that do not contain chlorophyll. Bark is not imperme-
able, but it is too thick to permit diffusion of oxygen into the
phloem and cambium beneath it; at a 21% oxygen concen-
tration in the atmosphere, oxygen could diffuse less than a
millimeter into the bark. This is why pores are necessary to
facilitate oxygen diffusion, at least to balance the oxygen
consumed by cellular respiration. This also is why oxygen
availability below ground in the root zone is important, and
why waterlogged soils that do not dry out can result in the
death of plants that are not adapted to these conditions of
oxygen limitation.
There are at least two different types of lenticels—
transverse and longitudinal—based on the orientation of the
lenticel opening along the stem. Longitudinal lenticels are
more involved in gas exchange than the transverse lenticels.
Under flooded conditions, the intercellular space of the layer
of cells beneath the lenticels, or phellogen, increases to facili-
tate the diffusion of gas. The phellogen look like raised white
bumps on the outer bark. Lenticels also affect transpiration by
being a portal of water-vapor escape (Kozlowski and Pallardy
1997). During the summer when leaves are out, most water
vapor is eliminated through the stomata, as leaves offer a
much lower resistance to water loss. After leaf drop, however,
the resistance to water-vapor elimination through the lenticels
offers a lower resistance to water movement.
Stems are a variable storage compartment of water but a
continual storage compartment of carbon (Chiou et al.
2001). For example, chemicals that have a greater octanol:
water partition coefficient, K ow , in a plant will be sorbed onto
tissue rather than become transpired; this is described further
in Part III.

The structure of leaves has to accommodate a variety of
processes; light capture, capture of CO 2 , formation of food,
transportation of food from the leaf, and be a site of water
exchange. These jobs are done by three primary tissues—the
epidermis, the mesophyll, and the vascular system. As in the
case of most epidermal layers of the plant, the epidermal
layer of leaves is one-cell thick and the outermost layer of
the leaf surface, save for the presence of wax on the cuticle
of the uppermost leaf surface. The lower epidermal layer
is broken up by the presence of the stomata. The guard
cells contain the chloroplasts. The mesophyll cells are
sandwiched in between the upper and lower epidermal
layers.
The vascular system is contained in the veins, which
probably is one of the more recognizable parts of a leaf.
Veins can run throughout a leaf. The narrow leaves of
grasses have veins that run parallel to the direction of the
leaf. Most other plants have a strong central vein fed by
numerous smaller secondary veins that spread out to the leaf
edge, and these are interconnected by smaller veins. The
central vein is attached to the petiole, which provides con-
nection to the stem or branch. These veins are locations
where the xylem and phloem are interconnected.
As the leaves form from the growing meristematic cells,
there is an overall shape to grow forward as well as cells that
will become the upper part of the leaf and cells that will
become the lower part of the leaf. Moreover, certain cells
will become the vascular tissues, some will become the
palisade tissue near the top surface, and some will become
the mesophyl near the lower surface. After most of this cell
division occurs, the leaf cells then can expand to the size of
their potential, without much additional division. Winter
buds, for example, contain encapsulated small leaves and
even flowers. Cellular division and growth have occurred
already even before the bud opens. All that then remains is
the expansion of these tissues by the uptake of water.
The function of leaves as sites of gas exchange is analo-
gous to mammalian lungs or fish gills, where O 2 is taken up
and CO 2 is released into fluids, such as air or water, respec-
tively. With lungs, every cell requires a fresh supply of
oxygen and must rid itself of CO 2 , but neither can diffuse
into or out of the cell to sources in the atmosphere. That is
the job of the circulatory system, which is the site of cell-to-
cell gas exchange with fluids and lungs where the gas
exchange with the atmosphere occurs. The mesophyll cells
of plants are in contact with air over 90% of their surface
area. This increases the diffusion of CO 2 into the leaves.
Water from the liquid phase is transferred to the air in a
vapor phase by diffusion. In general, an amount of water
equivalent to a leaf's fresh weight is vaporized every 20 min.
on a sunny day (Canny 1990). Leaves resist water loss in at
least three ways—the resistance to water-vapor loss in the
intracellular air spaces, IAS , connected to the stomata, r IAS ,

3.5.9 Leaves

The general shape of many plant leaves reflect a compromise
between light and CO 2 capture and water loss. All leaves can
restrict short-term daily water limitation by stomatal closure.
Longer term seasonal water restrictions, however, are not
always handled in this manner. Deciduous trees, for exam-
ple, shed their leaves in response to drier conditions.
Evergreens retain their leaves longer before shedding,
because they generally have fewer stomata, less leaf surface
area, and lower rates of water usage. As a tradeoff, however,
they tend to grow at slower rates than deciduous trees even
when water is not limiting.