Environmental Engineering Reference
In-Depth Information
resistance to water loss by the aperture of the stomata, r ST ,
(previously discussed), and even after water vapor escapes,
it enters the layer of saturated air, called the boundary layer,
next to the leaf surface, r BL . The total resistance to water
vapor loss from the leaf, l T ,is
the branches coming off the lateral branch and finally the
petioles from the branches. This is done by the plant to
maximize the amount of light and air available to all leaves
of the entire plant. This form is accomplished by the reaction
wood cells that arise from the live xylem. This can be seen in
examining the cross section of a piece of fallen limb; there
will be more wood, or xylem, on the underside of the branch
than on the top side. This is to provide structural support and,
in a way, acts as a cantilever.
The position of leaves with respect to each other also is
not random. Branches from the main trunk can be depicted
as lines coming from a circle, which would be the cross
section of the tree trunk. As you go up the trunk, successive
branches come off the circle at the same angle, be it 90 or
120 or another angle (Fig. 3.17 ). This succession of trunk to
branch is continued from branch to twig and from twig to
leaf petiole. Leaves can be arranged in an opposite pattern,
as is found in the maples, such as Acer rubrum . The arrange-
ment of leaves on a single stem or branch also maximizes
exposure to light; most leaves radiate outward from their
nearest neighbor.
Even the arrangement of stems from the main stem is at a
fixed angle and is designed to decrease the interference from
other stems for light capture. In fact, the angle between
branches and the main stem, between leaf petioles and
branches, and between small branches and large branches
tends to be the same for each species, a mathematical
l T ¼
r IAS þ
r ST þ
r BL :
(3.13)
In contrast to leaf resistance, the flow of water vapor can
be viewed in terms of the leaf conductance of water, which is
simply the inverse of the resistance, where leaf conductance
is 1/leaf resistance. The units of measurement are L/T (cm/s).
Leaf conductance increases with the amount of light avail-
able and with higher, less negative water potentials in the
leaves, but decreases when CO 2 concentrations are elevated
or the vapor pressure deficit is high. In this way, leaves can
be viewed as nozzles adjusted to such a fine spray that only
water vapor emanates from the openings.
Not only does the flat outer structure of leaves suggest its
role as the site of gas exchange, but so does the inner
structure. The cross section of a leaf reveals that while its
outer surface is protected from excessive water loss or other
liquid entry by the waxy cuticle, the side or sides with
stomata contains more void space than cells; this is the
place for gas exchange, with CO 2 dissolving in water along
a concentration gradient and water evaporating along a con-
centration gradient in response to the VPD.
The air space within leaves of various plants has been
estimated. In pine needles of conifers, air spaces represent
about 5% of the total volume of leaf tissue, and this tends to
increase with leaf size, such that corn is 10% and tobacco is
40%. Much like our lungs, the total surface area of these
pore surfaces may exceed the external leaf surface area by up
to 30% to ensure that gas exchange occurs.
Leaves often are covered with small projections of tissue
that resemble animal hair. These tissues, or trichomes (from
the Greek trichome , meaning growth of hair) are often found
on the underside of leaves. Each trichome is an individual
cell that arose from the meristem, and its rise above the leaf
surface is indicative of its rigid cell wall. Some trap air and
act to insulate the leaf from extreme drops in temperature.
On the other hand, trichomes can reduce the influence of
solar radiation and water loss by transpiration. Plants that
have trichomes have cells that contain the genes for such
structures, and this gene is absent in plants that do not
contain trichomes. Specialized leaf hairs also can directly
control the water status of plants. Such hairs that can release
water are called hydrathodes, and this occurs so that excess
water can be removed from the plant.
Close inspection of many woody plants reveals that a
characteristic angle of departure of leaves, as well as
branches, is repeated throughout the plant. The angle is the
same for the lateral branches from the main trunk and from
Fig. 3.17 The three branches of this small fern radiate from the main
stem at 120 (Photograph by author).
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