Environmental Engineering Reference
In-Depth Information
have holes or pits that permit exchange of fluid between
adjacent tracheid cells. Their construction is similar to how
perforated pipe laid in drains can move water along hydrau-
lic gradients both vertically and horizontally. This construc-
tion provides strength but at the cost of increasing the
resistance to water flow. Tracheids are more characteristic
of gymnosperms, such as conifers, although they also are
present in angiosperms. Angiosperms possess vessels, which
are not as narrow as the tracheids and are connected end-to-
end to form a continuous pipe; therefore, water flow is not as
impeded as with tracheids. Both tracheids and vessels con-
sist of cells that die after they form tubes; this is a conse-
quence of their intended use to transport fluids for the whole
plant rather than just within their own cytoplasm. Vessels
have rows of shorter cells that have thick cell walls on the
inside in the form of rings or spirals, similar to the rings
within the walls of mammalian trachea, or windpipe. These
cells cannot tap into the transpiration stream and place a
burden on the plant. Conifers have tracheids and resin canals
in the xylem. The xylem in pines are not hollow tubes
supported by spirals but instead are pitted. For example,
the xylem cells lined up end to end are not continuous, as
in the hardwoods, but pinch off to points at both ends, where
adjacent cells overlap. At this juncture, water can move
through the thin cell walls.
The xylem cells die each year as they are replaced by new
cells. The plant hormone auxin that is present in the cambial
cells regulates the growth (Tuominen et al. 1997) and death
(Moreau et al. 2005) of xylem cells. Higher auxin
concentrations are found closer to the cambium, and
concentrations decrease in the xylem cells with age. The
use of rings to document the life of a tree was first
documented by Leonardo da Vinci. He also observed that
the width of each ring was an indication of the relative
amount of moisture available during each year since growth
began.
Suction in the xylem can cause gases to enter the fluids,
called cavitation. Cavitation can occur due to excessive
tension, embolisms from air exchange with the cortex, freeze
and thaw cycles, or disease. Perhaps the most likely source
of cavitation is water limitation because of drought
conditions. Cavitation of the water column in the xylem
can decrease water transport and result in decreased hydrau-
lic conductivity, dehydration, and even plant death. Because
of the deleterious effect of cavitation on water transport in
the plant, and because water movement is induced by evap-
oration in the stomata, plants must maintain stomatal con-
ductance below the maximum that leads to cavitation
(Sparks and Black 1999). This is especially true in more
drought-resistant species, which have decreased hydraulic
conductivities and more control over stomatal conductance.
The presence of gases in xylem fluids is not just from
tension breaks but also from the entrance of atmospheric
oxygen and respiration production of CO
2
by surrounding
live tissues (Kramer and Kozlowski 1960). Both oxygen and
CO
2
can be present from 1% to 20% and typically are
inversely related. In most cases, the concentration of oxygen
is lower in the xylem than in the atmosphere, and the con-
centration of CO
2
is higher in the xylem because of the
presence of the respiring cambium. In the cortex and bark
outside of the cambium, however, gas content is similar to
that of ambient air by plant-atmosphere exchange through
lenticels.
Some xylem structures are considered adaptations to
lower water availability, much in the manner that stomata
are regulated by limited water or strongly negative water
potentials. For example, in areas of California that experi-
ence droughts, plants have high conductivity xylem when
water is available but narrower vessels and tracheids when
water is limiting. Such physiological adaptations are related
to avoidance of embolism formation, which would result in
plant death (Kolb and Davis 1994). Another survival
approach for deciduous trees is to replace embolized xylem
tissue each year with new xylem.
3.5.1.2 Phloem
The tissue that forms on the outside of the cambial stem cells
is similar to xylem in that it transport fluids, but that is where
the similarity stops. The cells that comprise the phloem are
relatively thin and connected end-to-end to form tubes that
permit the passage of water and solute, such as sugars, from
the shoots to roots and back again. They are present closer to
the surface of the plant than the xylem. The walls of these
cells are thinner than the xylem and, unlike the xylem, are
composed entirely of living cells and, therefore, retain their
cytoplasm. These phloem cells do not, however, contain
nuclei at maturity. The phloem consists of different cells,
depending upon whether the tree is a hardwood or conifer.
In hardwoods, the phloem cells consist of the sieve-tube cells
that are not entirely open end-to-end, and the companion cells.
As discussed previously, sap is a dilute solution of water
and organic molecules such as sugars. Sap also contains
proteins, such as amino acids, and hormones, such as
auxin, that are produced in the shoot meristems and travel
through the phloem to influence root growth in the root
meristems. The phloem and cambium of the Scotch pine
(
Pinus sylvestris
) was collected, dried, crushed, by
Laplanders and turned into bread during times of scarce
game, as was noted in 1732 by Linnaeus who traveled
throughout Lapland. The name Adirondack, commonly
associated with the mountain range in New York, actually
means tree eater, which was the custom of the Native Amer-
ican Adirondack tribe that lived in the Adirondack
mountains. These examples underscore the primary role of
phloem, to transport the products of photosynthesis through-
out the plant.
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