Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 88

dehydrate their leaves to reduce freezing. Deciduous trees
are more likely to be found in moist soils, but water avail-
ability is reduced when it is bound up as ice or snow. Hence,
leaf drop occurs as water availability declines.
Leaf drop also reduces evaporation from the upper layers
of the soil and provides organic matter that can be reused by
the plant after recycling by detritovores. One of the fastest
rates of such recycling is exhibited in the tropical rainforests,
where high temperatures, high precipitation amounts, and
poor soils cause a rapid recycling rate. Conversely, the
needle shape and thick bark of most conifers are attributes
that enable them to live in dry or cold climates.
Earlier in this chapter the interrelations among plant roots
and bacteria and fungi were discussed. There also is a rela-
tion between bacteria and the leaves of some plants. For
example, bacteria are present in the fluted margins of the
leaves of Ardisia crispa . These bacteria are heterotrophic
rather than photosynthetic and live in the apoplast area
between the cells. It is believed that they provide growth-
promoting substances because if the plants are heat treated to
kill the bacteria, the plants survive but grow at a reduced rate
with less vigor (Margulis and Sagan 2002). There also are
cyanobacteria ( Nostoc spp. ) that inhabit the leaves of
Gunnera manicata at higher elevations in Ecuador. Inside
the leaves, cyanobacteria reduce atmospheric nitrogen to
nitrate for plant uptake.
Many phreatophytic plants, either obligate or facultative,
have large leaf surface areas. This can be accomplished by
having fewer but larger leaves, as is characteristic of
Populus , or by having many smaller leaves, as is character-
istic of Betula or Salix . In both cases, the high rate of growth
of these plants requires the additional carbon to be supplied
by more fixation of CO 2 in the leaves, more water as a
reactant in photosynthesis, and increased gas exchange. It
is this total volume of tissue that undergoes gas exchange
that ultimately determines the size of a particular plant. As a
plant grows larger, the surface area must increase as a square
function. For example, doubling the radius of a sphere from
2 to 4 in. (5-10 cm) will result in a four-fold increase in
surface area and an eight-fold increase in volume.
A close analogy to the role that leaves play is given by the
solar panel. Both capture solar energy and convert it to other
uses (Fig. 3.19 ). Leaves absorb the sun's electromagnetic
radiation when it impinges on chlorophyll in chloroplasts
and the exited electrons are used to produce ATP from ADP,
essentially electrical energy. Solar panels take the same solar
radiation energy and convert it into electrical energy; for
example, a 4 ft 2 (0.37 m 2 ) solar panel can produce about
35 W of energy. Essentially, the sun is the source of
electrons and both leaves and solar panels distill these
electrons. Much like leaves, solar panels are thin and have
a large surface area. Solar panels consist of thin sheets of
semiconductor material, such as silicon, to which impurities,

Fig. 3.19 A leaf and a solar panel essentially perform the same task;
harnessing the sun's energy for storage in a more useable form. Here,
the solar panel provides power to a battery that runs lights for a parking
lot at night. The tree, a palo verde ( Cercidium microphyllum ) near
Tucson, Arizona, uses the sun to generate ATP to support life (Photo-
graph by author).

such as phosphate, have been added. The semiconductor
material is connected to metal contacts below the silicon.
Solar radiation through photons of light strike this material
and some of the energy is absorbed by the semiconductor,
which knock loose electrons from the silicon-phosphate
semiconductor, and this causes electron flow; this is why
solar panels are called photovoltaic cells. To direct this flow
into a current, electrical fields are used. The efficiency of a
typical solar panel is about 15%. Often, the sunlight energy
so obtained during the day is collected and stored in a battery
for use at night.
The local amount of sunlight across the United States
varies with latitude. Typically, there are about 4 h of peak
sunlight from Washington to Maine, and this number
increases farther south, which has 4.5-5 h of peak sunlight.
Maps of average solar radiation, called insolation maps, can
be used for a particular area and are discussed as part of site-
assessment activities presented in Chap. 6.

3.5.10 Vacuoles

Vacuoles serve many purposes for plant and water relations
as previously described, but also can be considered to be
the cellular dumping ground of the plant. As described in