Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 93

indicate whether or not water is present, even though it may
not be bioavailable. This is the main reason why tensiometers
are more useful and can rapidly indicate if water is at tensions
near to, equal, or greater than the wilting point.
Water potentials can be used to estimate the velocity of
water flow in the xylem. The velocity, W v , of water moving
upward through xylem of a constant radius, r , driven by a
difference in water potentials,

requirement for water to sustain life remains a common
denominator. Whereas the single-cell plant could float freely
about in the upper layers of surface water and be exposed to
sunlight, the multicellular plants had to develop a more
complex anatomical adaptation to secure the plant's needs
for water—the root system. Although underground, many
plants have extensive lateral roots that take in precipitation.
Even many of these plants, however, have tap roots that go
deeper into the subsurface to collect the more perennial
supply of groundwater in the event that precipitation is
infrequent or soils are porous. Hence, even though plants
have moved beyond being completely immersed in surface
water, terrestrial plants remain connected to water, and
phreatophytes primarily connected to groundwater.
Why is this information important to the phytore-
mediation of contaminated groundwater? The fact that
water is present is not as important to plants as is the
bioavailability of water—water has to be present at tensions
above the wilting point to be accessed by roots. Plants that
have roots that access groundwater, however, essentially
eliminate this physical constraint on water availability.

, measured between two
elevations that are separated by distance,

Dc

x , can be
estimated by this version of Poiseuille's equation:

D

W v ¼ð ð r Þ 2 = 8 viscosity ð Þ Þ Dc=D ð x Þ (3.18)

Water velocities between 1 and 45 m/h have been reported;
the low rates are for plants with small-diameter xylem, and the
high rates are for plants with large-diameter xylem.

3.7 Summary

From the landward transition of the earliest single-cell blue-
green algae to the complex multicellular land plants used for
the phytoremediation of contaminated groundwater, the