Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 61

organized chloroplasts. This provided an advantage over
algae because CO 2 is more readily available in air than in
water, even at its low atmospheric concentration of 350 ppm
(0.035%). Botanists classify mosses as bryophytes because
they do not have vascular tissues. Fossil evidence suggests
the presence of terrestrial bryophytes as early as 350 MYa
during the Devonian Period. They represent part of the
sequential steps toward vascular plants because, like the
water-conductive parts of terrestrial plants, or xylem, that
no longer are living cells and, therefore, lack cytoplasm so
that water can be transported at rates that exceed diffusion,
most mosses have cells that store water that also are no
longer living cells. That the adaptation of mosses to terres-
trial environments was successful is evident in their common
presence today.
In order to become what are recognized today as land
plants, several additional structural changes had to occur.
First, single cells had to group together into tissues specialized
to hold the plant upright at some distance above land surface.
This came about with the Psilophyta, which contained the first
true stems, characterized by the club mosses and horse tails.
The younger Carboniferous Period, some 300 MYa, was a
time of globally warmer temperatures and lush plant growth
that included large, tree-sized club mosses and ferns. These
plants formed thick deposits of peat after death and, following
burial and time, comprise the thick coal beds that we mine and
use today for energy. It is ironic, perhaps, that today the
burning of such fossil fuels is believed by many to be a
contributing cause of global warming, when, in fact, even
warmer temperatures than today were necessary to support
such extensive plant growth during the Carboniferous Period
to produce the fuels being burned. Moreover, it also is inter-
esting that the decayed plant remains that constitute the source
of most fossil fuels today consist of plants found in damp areas
near surface water and were, most likely, phreatophytes that
used groundwater.
Within this vertical support structure, other cells had to
align themselves to transport water from the soil to other
parts of the plant and to harvest sunlight while exposed to the
air; this later process required leaves. The first true leaves
were found in the Filicophyta , or ferns. These structures
provided more surface area for gas exchange to occur than
that offered by stems alone. It is these structural changes that
resulted in the classification of vascular plants. The exposed
epidermal cells had to synthesize compounds that would
render them waterproof against loss of moisture, but, at the
same time, provide some permeability to permit the entrance
of CO 2 . Fossil evidence indicates the rise of vascular plants
at least from 400 MYa, during the Silurian Period. The fossil
evidence includes one of the earliest vascular plants, Rhymia
major , which had a separate set of cells shaped in a cylinder
inside the body that permitted water to move upward and
downward inside the plant.

This evolutionary progression from aquatic, single-celled
algae to multicellular, vascular land plants is repeated today
during the colonization and subsequent succession of barren
land by plants. For example, rocks exposed at land surface
originally contain no topsoil, so it is essentially devoid of
moisture and available nutrients and, therefore, any life
forms. Under such conditions lichen thrives, which are
both algae, typically blue-green, and fungi. The lichen
survives by extracting the bound nutrients from the rock or
barren soils of abandoned fields by using acidic excretions.
Mosses become established on the detritus left behind by the
lichens and then, when sufficient accumulations of organic
matter are deposited, seeds from annual vascular land plants
with high rates of seed production, dispersal, and germina-
tion are established. Once a fertile soil is developed, and soil
moisture and groundwater accumulate, slow-growing vascular
plants, such as conifers, dominate, and then fast-growing
deciduous trees. The result of such succession is the devel-
opment of a hardwood forest ecosystem, often called the
climax community, and may take many years. In many
ways, the ecological succession of plants, one on top of each
other, is similar to the development of human culture and
civilization in the same area over time; across the Middle
East, for example, there are many nongeologic topographic
highs, called tells, which represent the succession of invading
cultures that then built upon the wreckage of the invaded.
The reference to seed production, dispersal, and germina-
tion above gives rise to another chapter in the history of plant
evolution; the earliest plants did not reproduce sexually, but
asexually, without flowers. The link of current plants to an
aquatic past is reflected in the need for lichen and mosses to
have enough moisture available, usually supplied by dew, to
permit the sperm to swim to the ovary of the lichen or moss.
These non-flowering plants included the mosses and ferns,
which reproduce using spores. Later, the inefficiency of
spore production compared to germination led to seed pro-
duction by these and other plants, such as cycads and
conifers. The seed is a fertilized egg surrounded by a source
of food and protected from the environment by a tough seed
coat. These seeds, because they arose without a flower, are
called gymnosperms. The pines today are relatively
unchanged from those of 300 MYa. The development of
pollen as a means of transporting genetic information was
first seen in gymnosperms. Although pollen is an irritant for
allergy suffers, it marked a profound step forward to the
terrestrial habitat for plants, because it freed plants from
relying on a watery medium for reproduction. Plant success
on land is not only a consequence of the advantage of seed-
based reproduction and the absence of water as a vector of
fertilization, as is necessary for mosses, but also, as we will
see later in this chapter, the advantage of structural
adaptations to survive in environments where water is a
limiting factor.