Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 60

of some of these roots exposed in caves. These researchers
reported that deeper roots have wider diameters in xylem
tissue than shallower roots from the same plant, perhaps in
order to overcome increased hydraulic resistance at depth
(McElrone et al. 2004)—this is similar to the need for
thicker electrical wiring as its length increases in order to
minimize frictional losses of electron flow. In other words,
because water absorbed by deeper roots has a longer path to
travel than water absorbed by shallower roots, the diameter
of the deeper roots must be larger to overcome limitations
imposed by resistance to flow.

diffusion over the length of the body of an aquatic plant, it
is restricted to the subsurface root zone that often remains
moist, especially for phreatophytes with roots in the capil-
lary fringe or water table.
The multicellular forms of green algae may have evolved
from the tendency of unicellular algae to attach to each
other. These algae are found in both freshwater and saltwa-
ter. Green algae, which contain chlorophyll a and b as well
as the accessory pigment betacarotene, for example, is found
in many forms, but one of the simplest is the attachment of
cells end-on-end to form strings of cells of variable length.
These algae evolved from a common uralgae, the first
oxygen-producing plants that started turning the originally
reducing atmosphere of the earth into an oxidizing one. The
interrelation of these cells is more than simply a grouping of
independent cells, which technically would be considered a
colony, but rather the reproduction of cells that have inde-
pendent cell walls that remain attached following division.
For example, the freshwater green algae Spirogyra , familiar
to most primary-school biology students, has some division
of labor among cells. This is clearly indicated by the forma-
tion of a holdfast on the last cell used to anchor the algae and
is considered to be a protoroot. Another example is seaweed,
or sea lettuce, called Ulva , which is commonly found on
beaches along the northeastern United States.
The initial transition from aquatic to terrestrial plant life
probably started when some algal cells remained exposed
after being washed up onto a shoreline but resisted drying
out completely because their epidermal layer contained wax
and the other parts of the epidermis contained holes that
permitted the introduction to the cell of atmospheric, rather
than dissolved or aqueous, CO 2 . Alternatively, algal cells
could have been washed up and then covered with a fine
layer of sediment that protected them from desiccation. In
either case, this transition occurred about 500 MYa (million
years ago). The capability of cells to prevent desiccation and
to essentially bring water with them as they colonized the
landscape was an important development that led to the
colonization of land by plants. Moreover, this ability to resist
drying out provided a selective advantage, because it
provided access to minerals in the soil matrix where compe-
tition was less intense than in water due to the vast numbers
of algae and phytoplankton.
Mosses represent the next evolutionary step from aquatic
algae toward the establishment of terrestrial plants.
Although moss clumps, or beds, look like a single plant, it
is actually composed of many thousands of closely spaced,
single-stranded plants. Each strand, or filament, has
structures that grow down into the soil for anchorage, called
rhizoids—again, a type of protoroot. The green part above
ground has scales that function as leaves, although they
technically are not, that also can absorb moisture from the
air. Mosses probably were some of the first plants to contain

3.2.2 Root Evolution

A discussion of roots, their function, relation to water use,
and relevance to phytoremediation cannot be considered
adequate without some background on the evolutionary his-
tory of terrestrial plants. The common theme between plant
evolution and phytoremediation is how plants obtain the
necessary elements from the environment for survival and
reproduction. Much of this information is based on fossil
and molecular clock evidence; molecular, or gene, clock
determination uses the constant rate of change in DNA
mutation caused by random drift to understand evolutionary
development.
Terrestrial plants, such as those used for phytore-
mediation purposes, began as unattached, one-celled photo-
synthetic organisms, such as algae, that floated near ocean or
freshwater surfaces. Algae include both unicellular
organisms, such as the blue-green algae, and multicellular
organisms, such as green algae. Most contain chlorophyll
and, therefore, are photosynthetic and considered to be
aquatic plants. These are not true plants, however, because
they do not contain cells organized into tissues or organs,
such as stems and leaves, nor can they reproduce sexually
like higher plants. The unattached unicellular blue-green
algae bask in freshwaters near the surface to obtain sunlight
and have multiple sources of CO 2 to sustain photosynthesis,
including atmospheric CO 2 , dissolved CO 2 as bicarbonate
ions, and CO 2 as minerals. Under such conditions a vascular
system is simply not necessary.
The transition of plant life from water to land could have
occurred only through the adaptive formation of structures
that provided selective advantages to these early plants for
survival. Plants went from structures that encouraged gas
and water exchange by diffusion, as seen in green algae, to
structures that resisted diffusional water loss, except at
highly regulated locations, such as the stomata. Plants went
from the lack of a rigid support system, as none was neces-
sary in the water which supported the plants, to the develop-
ment of internal woody fibers that provided rigidity even
after the cells died. Whereas water absorption occurs by