Environmental Engineering Reference
In-Depth Information
3.5.7 Compensating Pressure Theory
The observation that embolism damage can be repaired and
transpiration resumed (Canny 1997) places in doubt the
Cohesion-Tension Theory of transpiration. Canny (1997)
showed that breaks in the transpiration stream and
subsequent repairs may happen to a plant several times a
day. In fact, the very tension that Scholander et al. (1965)
reported is regarded by Canny (1997) to be the compensating
pressure applied to the embolized vessels.
3.5.8 Stems, Bark, and Lenticels
When one looks at plants closely, it becomes apparent that
for most plants, all parts of their structure are derived from
modified leaves, as was written about by the poet and natu-
ralist G
oethe. There are many similarities between the func-
tion of leaves and stems. Stems are the above-ground
extension of the root neck that provides the support structure
for the leaves. Stems contain the vascular tissues for sugar
and water transport. However, like leaves, some stems con-
tain chlorophyll and can undergo gas exchange and photo-
synthesis. For example, cacti and other succulents carry out
photosynthesis not in their leaves, which have been reduced
to spiny projections, but in their stems, as a consequence of
low water availability and high vapor pressure deficits. In
some cases, more food is produced than consumed in some
herbaceous plants with green stems, such as sugar cane.
Additional gas exchange can occur from the atmosphere
to the cambium through the lenticels (Fig. 3.16 ). Here,
oxygen can diffuse in and drive cellular respiration, espe-
cially in the growing phloem and cambium. Lenticels also
have been shown to be the source of adventitious root devel-
opment in some phreatophytes (Ginzburg 1967). If the stems
of such plants are exposed to an increase in water content,
such as submersion during flooding or increased air humid-
ity, the parenchymal cells fill with water and expand and
then grow and divide, sometimes producing multiple roots
from one lenticel. It is the genes of leaf cells that give rise to
other types of cells to form other tissues.
Branches are secondary stems that depart from the main
stem. These secondary stems produce additional branches
and contain leaves and buds. This growth is called ramifica-
tion. If the main stem continues its upward growth through-
out the plant's life and lateral stems grow from it, this is
termed monopodial ramification. This is characteristic of
most conifers. On the other hand, deciduous trees tend to
stop growth in the main stem, and the lateral stem growth
predominates. This is called sympodial ramification.
Bark is the outermost layer of cells of woody plants. It is
produced by the cork cambium beneath it, and it grows
outward and then dies. Bark consists of non-living (no
Fig. 3.16 Lenticels on a 3-years old hybrid poplar tree at a
phytoremediation site near Charleston, SC (Photograph by author).
metabolism) cells that are filled with lipids. Crevices created
by the expansion of the underlying and growing cambium
are used to supply oxygen to the growing and respiring cells
through the lenticels. One of the purposes of the bark is to
protect the growing cambium from environmental assault
from the outside and to prevent the loss of sap and water
from the inside. It tends to be more waterproof than water
permeable. One example of the excellent waterproofing
traits of bark is provided by the birch tree, which was used
for thousands of years by the native people of North America
in the construction of canoes. The bark typically was gath-
ered during the summer and, after removal from the tree, was
placed inside out on the canoe's frame.
Following winter leaf drop, the effect of stems on tran-
spiration can be substantial. This is caused by the lenticels.
Much as humans loose their outer layer of skin cells period-
ically, so do trees loose their outer layer of cork. The
lenticels allow oxygen to reach the cells in the growing
and, therefore, respiring layer of cambium cells beneath
the bark and cork, mainly as diffusive transport through the
intercellular space in the cortex. Because of the presence of
the continuous cambial sheath, however, it is unlikely that
gas exchange occurs between the outer phloem and inner
xylem by way of the lenticels. The cork cambium cells that
do not contain suberin can extend through the periderm to
the atmosphere. They are similar to the stomata in that they
are sites of gas exchange. After all, plants are aerobic
organisms and require oxygen in order to respire the food
they make, and this process occurs in all cells. Many plant
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