Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 44

sufficient counterbalance to it, and that the remedy is adequate
to the evil.
Joseph Priestley, Experiments and Observations (1774)
(from Kramer and Boyer 1995)

reacts faster than the heavier and kinetically slower isotope
( 18 O).
This difference in oxygen isotope mass and kinetics was
exploited by Samuel Ruben and Martin Kamen, who
applied water that contained the heavy oxygen isotope
(H 2 18 O) and CO 2 that did not (C 16 O 2 )toplants.They
then measured the isotopic composition of the oxygen
released by the plants and found that it contained the
heavy isotope ( 18 O), thus confirming that the oxygen
released by plants is derived from water. Hence, the impor-
tant role of water in the photosynthetic equation in the
creation of organic compounds from inorganic compounds
was demonstrated. Moreover, groundwater can be an
important source of water used in photosynthesis. This
fact provides a rational basis for the use of plants that
interact with groundwater at sites characterized by ground-
water contamination. First, however, water entry must
be discussed, and this means looking at plant-water
interactions at the cellular level.

A twist in this story is that prior to Priestley's experiment,
the Swedish chemist Carl W. Scheele also discovered that
oxygen was the gas given off by plants, but he is less
recognized today because his paper was published after
Priestley's paper.
In 1779, a colleague of Priestley, the Dutchman Jan
Ingenhousz, demonstrated in Experiments on Vegetables
that the mystery element must be a gas, because when he
placed a plant under water the leaves, but not the stems,
released bubbles; this gas production was observed only
when the leaves were exposed to sunlight, and gas bubbles
were not released in the dark (Leicester and Klickstein
1952). He also demonstrated that plants release CO 2 in
addition to oxygen and can use it during photosynthesis,
which will be discussed later.
In 1780, the pastor Jean Senebier observed that only the
green parts of plants that contained chlorophyll released a
gas which we now know to be oxygen, and took in gaseous
CO 2 . This helped to confirm Ingenhousz's observations of a
lack of bubble production by stems. The Swiss botanist
N. T. de Saussure, introduced in Chap. 1, showed that the
release of oxygen by plants occurred only if the uptake of
CO 2 occurred. In 1844, Hugo von Mohl discovered what he
called leaf green or chloroplasts, in the green parts of plants.
Finally, in 1845, the German biologist Robert Mayer wrote
that plants absorb one form of energy as light and give off
another form of energy as chemical bonds.
It may appear that all the parameters in the process of
photosynthesis are accounted for. However, what remained
unanswered was the source of the oxygen released. Was the
oxygen from gaseous CO 2 or liquid water, both of which
contain oxygen? Because it was recognized that oxygen also
is a gas, it was assumed that the oxygen produced was
derived from gaseous CO 2 rather than liquid water. More-
over, because two atoms of oxygen compose the oxygen
molecule, the implication is that two molecules of water
would be required.
In the 1930s, this question was examined by C.B. Van Niel
who proposed that water, not CO 2 , was the source of oxygen
released by plants. Later, the different stable isotopes
of oxygen were used to confirm Van Niel's hypothesis.
As will be shown in Chap. 9 oxygen stable isotopes are useful
in many studies involving water, because oxygen has a light
isotope ( 16 O) and a heavy isotope ( 18 O) that consists of
two additional neutrons. This difference in the number of
neutrons results in a difference in the atomic mass of each
isotope, and each isotope behaves differently in chemical
reactions. For example, the lighter oxygen isotope ( 16 O)

3.1.2 Prokaryotic and Eukaryotic Cells

Robert Hooke's observations of cork using a crude micro-
scope revealed that the cork was not a homogeneous solid
mass but composed of small chambers separated by walls
that Hooke called cells, as described in Chap. 1. The idea
that any organism or object was composed of cells was
revolutionary. Hooke's observation led to the formation by
Matthias Schleiden in 1838 of the Cell Theory, stating that
all life consists of cells, and a single cell is the minimal unit
that has the properties of life.
The Cell Theory also provides a framework to explain
the structures within cells and their functions. Anatomy is
the study of the structure of living things. From an
anatomical basis, plants can be conceptualized as a fluid
(liquid) that lives in another fluid (gas) above ground, and
below ground lives in both fluids. Each interaction between
these fluids and the plant is separated by membranes. Phys-
iology, on the other hand, is the study of the function of
these structures. Physiology tries to answer such questions
as why is a leaf shaped the way it is, or why does one plant
have a shallow root system and another plant has a deep
root system?
Plants are composed of individual cells with groups of
specialized cells that make up various organs, such as roots,
stems, and leaves, each which have different functions. This
is similar to most multicellular organisms. All plant cells
are alive, or in the case of xylem cells were once alive,
and work together to satisfy the needs of the plant as a
total organism.