Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 62

The flowering plants are called angiosperms, or enclosed
seeds, and they can be found in the fossil record as far back
as the end of the Cretaceous Period some 65 MYa. These
plants are divided into monocots and dicots, as was
discussed in Chap. 1, based on the number of leaves, either
one or two, respectively, attached to a seedling. In general,
the dicots have highly differentiated vascular systems that
formed during secondary growth from the meristem,
whereas monocots have more random and diffuse systems
and no secondary growth; any increase in girth is caused by
cell elongation or enlargement, and not division.
Some plants have returned to an aquatic habitat after a
period of land colonization, and some highly developed
plants never fully left the watery environment in the first
place. One such type of aquatic plant is the water lily, in
which the root, or in this case the rhizome or underground
stem, is submerged in the bed sediment of a lake or pond, the
stem is in the surface-water column, and the leaves and
flowers float on the water surface exposed to sunlight and
air. Air diffuses along a concentration gradient in the cortex
cells or aerenchymal tissues to the root zone to support root
respiration even in sediments that often do not contain oxy-
gen. Water lilies produce flowers and, therefore, had once in
the past made the transition to land but arrived back in the
aquatic environment.
Some plants live entirely under water, such as elodea or
hydrilla . These plants contain extra amounts of chloroplasts
to deal with the dilution of light in the water column and
have large air spaces in their tissues. They obtain water by
direct diffusion into the leaf cells and the CO 2 from the
atmosphere or bicarbonate. The roots of such plants do not
have vascular tissues for water transport because they lack
xylem and, therefore, the need for root hairs, but they do
have phloem for food transport and storage and a root for
anchorage. Other plants float about on the surface of the
water without attached roots; these are truly hydroponic
plants, such as duckweed ( Lemna spp. ), which essentially
is a free-floating leaf.

roots can become the main tap root, due to water limitations,
infestation or animal attack, or impermeable geologic strata.
Adventitious roots often can be short lived, however, and
thrive only if abundant moisture is available. A cutting of a
poplar tree placed in a plastic bag, for example, that contains
high humidity will form adventitious roots (Fig. 3.7 ), but
these roots will shrink and die after the cutting is exposed to
drier air. The formation of adventitious roots on stock used to
install phytoremediation systems and its implications for suc-
cessful planting is discussed in more detail in Chap. 7.

3.2.4 Root Hydraulic Conductivity

Hydraulic conductivity is a term that describes the charac-
teristic of fluid movement through a porous media and is,
somewhat fortuitously, common to both plant physiology
and groundwater hydrology. With respect to plants, hydrau-
lic conductivity describes the diffusive movement of water
from one cell to another cell through the cell membranes and
plasmadesmata, called the symplastic pathway. The central
question here regarding the flow of water from cell to cell is
what the initial rate of water movement is and what controls
this rate. The resistance to water movement by the cell
membrane of roots can be quantified and is called root
hydraulic conductivity, L p . As we will see in Chap. 4,
when used in groundwater hydrology, the term hydraulic

3.2.3 Adventitious Roots

Roots also can develop above ground and are called adventi-
tious roots. Adventitious roots arise from the root initials
located in or near lenticels along the stem (Ginzburg 1967;
Hook et al. 1970). They are found especially in riparian
facultative phreatophytes that grow in areas where surface-
water levels and sediment elevations fluctuate as is character-
istic of a flooding regime. Adventitious roots have vascular
connections that terminate within the annual growth ring of
the tree's current year. Most adventitious roots arise under
conditions when water is not limiting, whether from periodic
flooding or high humidity levels. In some cases, adventitious

Fig. 3.7 Adventitious roots on hybrid poplar cuttings formed during
shipment in plastic bags and elongated after immersion of the cut end
into water (Photograph by author).