Historical Foundation of Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 11

Swiss botanist N.T. de Saussure (1767-1845) found that the
absorption of certain minerals by roots was not proportional
to the absorption of water. Moreover, he determined that
these solutes were not absorbed in proportion to their
occurrence in soils, which suggested that roots have a
selective permeability that allows entry of different solutes
differentially.
The entry of water and solutes into plants was further
investigated by H.H. Dutrochet in the 1830s, who explained
the entry of solutes into plants as being driven along
gradients in solute concentration. Known as osmosis (from
the Greek osmos , meaning thrust or push) this provided a
firmer physical-based process to explain water uptake by
plants. This idea persisted for most of the nineteenth century.
In fact, Charles Darwin investigated the relation between
roots and the uptake of various solutes, such as ammonia
carbonate, which resulted in the deposition of brown
granules in the endodermal cells of plants, as reported for
Euphorbia peplus (Darwin 1882). In the late 1800s, J. von
Sachs offered the concept that the loss of water from leaves
above ground, or transpiration, was related to water uptake
in the root zone below ground. He also determined that clay
soils contain water more available to plants than sandy soils,
an observation that indicates that perhaps von Sachs was
familiar with some of the fundamentals of hydrogeology,
such as permeability, porosity, and hydraulic conductivity.
In the early 1900s, the idea of osmotic pressures as the
metric for water status in plants was beginning to be
challenged by the concept of water potential. This challenge
arose from measurements of osmotic pressures that could
not explain the movement of water throughout an entire
plant. Additionally, the devices used to measure water
potential became more accurate than the devices used to
measure osmotic pressure. Today (2011), the generally
accepted theory of water uptake by plants is that water is
passively taken in as a consequence of the evaporation-
driven process of transpiration from leaves along water-
potential gradients;

that compose the opposite end of the water spectrum, that is,
plants that grow in the absence of constant supplies of water,
are called xerophytes (from the Greek xero meaning dry).
Other plants that can inhabit an area with available water
but also high levels of salts are called halophytes (from
the Greek halo meaning sea). As would follow, the
xerohalophytes inhabit drier areas with high salt content,
such as near evaporite deposits. The plants called
mesophytes (from the Greek meso meaning middle), grow
well in moist soils that are sufficiently aerated and use
precipitation, when abundant, or groundwater during less
frequent precipitation. The generalized root distribution of
these plants and their relation to groundwater is shown in
Fig. 1.4 . Finally, observations made in the early twentieth
century by a then young hydrogeologist, who we will meet
soon, gave rise to the term phreatophyte (from the Greek
phreato meaning well or well plant) to plants that remove
water from the capillary fringe or water table. Plants that are
totally dependent on groundwater are called obligate
phreatophytes, and those that can use other water sources
are called facultative phreatophytes . Perhaps this classifica-
tion scheme can be viewed as the beginning of the integra-
tion between plant physiologists and hydrogeologists.
Finally, W.A. Cannon, a botanist with the Desert Botani-
cal Laboratory of the Carnegie Institution of Washington,
located in Tucson, AZ, made this statement:

The problems which deal with the presence of trees are primar-
ily physiological and have mainly to do with the absorption and
conservation of water. Each of these capacities varies with the
species. Of the root relations, that of the root-and-water table is
of prime importance, owing to the fact that the soil horizon
tapped by the roots of trees derives by capillarity, from the
level of the groundwater, its perennial supply of moisture.
W.A. Cannon (1923)

We will see how insightful about hydrogeology this bot-
anist turned out to be.

these concepts will be discussed in

1.2 Hydrogeologic Contributions

Chap. 3.
From this short history of the study of plants and their
interactions with soil and water, it is evident that the source
of water taken up by plants did not concern early plant
physiologists. However, later plant physiologists did come
close to making such a distinction when they developed a
generalized system of plant classification based on the
apparent source of water used by some plants. Early
botanists recognized that certain types of plants could only
be found when certain sources of water were present. This
observation often is reflected in the common names of
plants, such as bog asphodel, pondweed, and water lily.
Plants that consistently are found in areas of constant water
are called hydrophytes (from the Greek hydro and phyte
meaning water plant)—these are the aquatic plants. Plants

During the late nineteenth century when water potential was
replacing osmosis as the metric for water studies in the field
of plant physiology, the western part of the recently re-
United States was being settled following the Civil War. It
quickly became apparent that the largest constraint on poten-
tial settlement of this arid area was the lack of large
quantities of surface water that were more prevalent in the
humid eastern states. The need to examine alternative
sources of water provided the necessary economic impetus
to study the occurrence of alternative water sources, such as
groundwater.
The study of groundwater occurrence and availability
during this time was relatively new. So new, in fact, there