Environmental Engineering Reference
In-Depth Information
groundwater may play a role in the maintenance of tall
redwoods over 2 millennia.
The transport of water to the top of plants cannot be solely
explained by root pressures that result from the osmotic
uptake of water by roots, as was discussed previously. In
fact, the pressures of water within the xylem have to be less
than atmospheric pressure in order to follow a decreasing
pressure gradient from root to leaf. Because atmospheric
pressure tries to enter a vacuum, it is possible that water is
moved upward as a vacuum is created inside the xylem.
However, the maximum lift generated by such a process
would be no greater than 33 ft (10 m) at sea level, the weight
of the atmosphere, and would decrease at higher elevations.
The aquatic plant royal water lily ( Victoria amazonica ) has
very large leaves, as much as 8 ft (2.4 m) across, and grows
from a rhizome located on the bottom of a pond. A petiole
connects the rhizome to the leaf at the water surface. This
petiole can approach lengths up to 22 ft (6.7 m) but has not
been found to be longer, presumably because of the inability
for evaporation to pull a vacuum greater than the atmo-
spheric pressure that holds it down.
In the nineteenth century, it was thought that the plant
cells acted as small pumps to push the water up to the leaves.
This hypothesis was dispelled, however, by the German
botanist Eduard Strasburger after he killed the cells of a
40-ft (12 m) length of wisteria vine by boiling them but
left the upper leaves alive. The leaves did not wilt and die,
which indicated that water transport did not require the cells
in the stem to be alive. Strasburger observed a similar
response after he immersed the stems of plants in toxic
solutions of heavy metals. The conclusion was that living
forces were not necessary to raise the water to great heights.
The amount of water transpired relative to the amount of
water used to produce biomass can be described as the
transpiration ratio of a particular plant. This transpiration
ratio, TR , is the weighted value in mass of water used to
produce a mass of a particular crop. A TR of 1 indicates that
all water taken up by the plant was used to produce biomass,
an unlikely scenario. A TR greater than 1 indicates that extra
water was taken up but not used to support biomass synthe-
sis. Corn, for example, grown in Colorado has an average
transpiration ratio of 1,405 lbs (638 kg) of water per pound
of crop (Van der Leeden et al. 1990a, 1990b). Watermelons
have an average TR of 1,102 lbs (500 kg) of water per pound
of crop. On average, more than 2,000 tons of water must pass
through the roots of a crop plant to result in only 20 tons of
biomass (1%), and even after the crop is dried, only 5 tons
will remain, of which 3 tons will be from water. Hence, only
0.15% of the total water used is actually incorporated into
the plant biomass; the remainder is returned to the atmo-
sphere by ET . Of this ET , the proportion that is derived from
groundwater may approach 100% in dry climates (Moreo
et al. 2007).
Transpiration ratios exist for other plants as well. As
might be expected, many native weed species in North
America can tolerate drier conditions and have TR s under
500. Transpiration ratios also are available for woody plants
that may be used at phytoremediation sites. Because trees are
not considered to be crops in the same manner as corn, the
TR is computed as the pounds of water needed per pound of
dry-leaf matter. Maple trees that thrive in moist soils have a
TR of 1,281 whereas conifers have TR s less than 250 (Van
der Leeden et al. 1990a, 1990b).
3.5.1 Xylem and Phloem
We have seen how the cellular structures of plant tissues
relate to water uptake at the level of the individual cell.
Liquid water enters individual root hair cells by capillary
action and osmosis, diffuses to the xylem, and rises to the
leaves against gravity to exit as water vapor. But how
exactly are the two phases of water connected in a plant?
Part of the answer lies with the transition from the oceans
and lakes of one-celled photosynthetic algae to the land.
Successful transition resulted only after a set of inter-
connected cells was evolved to transport water and it's
dissolved constituents under negative pressure. As stated
previously, lichens and mosses do not have such conducting
tissues, whereas ferns do. Ferns contain substantial amounts
of lignin that enable them to stand erect off the ground in
search of light and provide protection from desiccation and
herbivory. Ferns grew to great heights in primeval forests
when the atmosphere was warmer than today. Currently,
ferns are much shorter and tend to be present as understory
plants in temperate forests. Ferns still need water to repro-
duce, however, as a medium to transmit sperm to the ovule.
It is the linkage between cellular structure and the laws
that govern the physical properties of matter that interact
together in the movement of fluids in vascular plants through
the xylem and phloem. As we will soon see, these tissues are
present from the growing root tip to the stems to leaf petioles
to the leaf veins.
3.5.1.1 Xylem
The xylem is composed of thick cells connected end on end,
thereby forming continuous tubes that allow the passage of
water from the site of entry in root hairs to the leaves. Xylem
is produced by the vascular cambium stem cells on the side
facing the center of the plant and only function to carry water
after they have died and lost their cytoplasm. The cambium
is the 2 to 10-cell thick layer of living tissue that gives rise to
the xylem and phloem in dicots. It is absent in monocots that
have only primary growth.
The xylem is composed of tracheid cells and vessel cells.
The tracheids are essentially single, long, pointed cells that
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