Environmental Engineering Reference
In-Depth Information
Chap. 11, vacuoles aid in the survival of a plant when
exposed to changes in the geochemistry of the soil or water
it encounters. For example, although calcium is necessary
for plant growth, excess calcium can be eliminated as cal-
cium oxalate in specialized vacuoles called crystal
idioblasts. Calcium oxalate crystals in plants were observed
as early as the seventeenth century, by Antonie van
Leeuwenhoek. Calcium oxalate crystals are perhaps best
known by people afflicted with kidney stones, which are
hard crystals of calcium oxalate. The source is often a diet
rich in plants, such as spinach, which contains these minerals
in the vacuoles. The formation of these crystals is thought to
impart a wide range of advantages to a plant, from defense
against herbivores, by a form of crystals called raphides,
which are discussed in Chap. 11, to calcium regulation and
structural support.
included here because it indicates the importance of the
interaction between plants and sources of water.
3.5.12 Dew
Water found on plant leaves early on a cool morning after a
previously hot, humid day is called dew. As the ground cools
faster than evaporation occurs, water in the atmosphere
condenses. One source of this water can be from plant
transpiration during the previous day or night. Although
the formation of dew recaptures some of the water that
would have left by evaporation, it is a source of water that
is typically important only in very arid climates. Lichens,
however, use dew as a water source in most climates.
3.6
Plant Water Status
3.5.11 Plant Life Forms
Water can compose up to 90% of most plants. To briefly
summarize what was discussed previously, water is used as
the solvent in the cell cytoplasm, the source of hydrogen in
the carbohydrates made by photosynthesis, a vector of entry
into cells and movement throughout the plant, and provides
support and cell elongation. Water entry into cells by osmo-
sis occurs when the concentration of water inside the plant
cell is lower than the concentration of water outside the cell.
Because the osmotic potential of cells varies, an alternative
measurement of plant water status is desirable; this parame-
ter is called water potential.
Even casual observation reveals the role that the location and
environment have on predominant plant life, distribution,
and diversity. The establishment of monocultures across a
wide area is the exception, such as
Spartina
or
Mangrove
or
a cultivated field, rather than the rule. But even in an area of
varied plants, certain distributions occur. The distribution
can be one affected by space (location) and time (season).
For example, some trees in the same forest lose leaves in the
fall and some do not. One of the earliest systems of
classifying this distribution of plants was performed by the
Danish botanist Christen Raunkiaer in 1903, and is based on
the occurrence and position of buds relative to ground sur-
face. It contains five different classifications:
3.6.1 Water Potential
1.
Phanaerophytes,
where the buds are 9.8 in. (25 cm) above
ground surface and include most trees, shrubs, and vines
2.
Chamaephytes
, where the buds are closer to the ground,
below 9.8 in. (25 cm), and include the herbaceous and
some woody plants
3.
Hemicryptophytes
, where the top growth dies but a bud
persists at or below the ground surface, such as grasses
4.
Cryptophytes
, where the bud is beneath the ground sur-
face or water; and
5.
Therophytes
, where the bud is not present but the plant
persists by seeds.
A Sisyphean task is one in which just as the task is about to
be completed, everything falls back to the level at which the
task was started. Its derivation is from the Greek myth where
Sisyphus, as part of his punishment by the god Zeus, was
tasked with pushing a heavy boulder up a hill, only to have it
roll back down the hill as he approached the top.
Although it may be a useful metaphor for some aspects of
life, it also is a useful way to describe the energy contained
in matter based on its relative position. In the example
above, the elevation of the hill provided the potential energy
for the boulder, which was turned into kinetic energy as the
boulder rolled downhill. Water can assume similar potential
energy in an elevated storage tank which is released as
kinetic energy when a garden sprinkler or water fountain is
turned on.
The first law of thermodynamics states that energy may
be transferred but the sum remains constant or is conserved:
energy can be changed but not destroyed. An excellent
example is the internal combustion engine, which takes
The main variable here is the extent of protection that
different plants offer to the buds to ensure survival of the
next generation. This is affected directly by the environment
and climate of an area. In humid areas that do not have
frost, for example, the plant communities are dominated by
Phanaerophytes. Conversely, desert areas of limited mois-
ture typically discourage such plants in favor of Therophytes.
Although this classification scheme is not widely used, it is
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