Environmental Engineering Reference
In-Depth Information
1.1.6 Stephen Hales and Fluid Pressure
in Plants
A unique feature about the water-transporting xylem cells
previously mentioned is that after their differentiation and
growth, these cells die, so that the entire plumbing system
necessary to keep a tree alive is composed of non-living
tissues. Plants can, therefore, transport water using cells
that do not require an energy supply. Also, water transport
is not constrained by diffusion because adjacent cells walls
are not present in the non-living xylem. Although the trans-
port of water in the xylem requires no expenditure of energy
by plant cells, the accumulation of ions in the roots as water
is taken up from soils lowers water potentials (i.e., the water
concentration) in the root cells and does require the input of
energy by living root cells. These topics are described in
detail in Chap. 3.
The question of how far water can move up the xylem
against gravity was an intimidating one for scientists in
the seventeenth century. A clergyman and physician named
Stephen Hales (1677-1761) made the first direct measure-
ments of the pressure of water in the roots of plants. He, like
other contemporaries such as Harvey and Malpighi, had
been investigating the flow of blood in mammals and won-
dered if fluid flow in plants also followed the model of closed
circulation. Hales used a piece of bladder, from a goat
perhaps, tied around a pruned grape vine to stop its loss of
fluid at the cut and realized the bladder could be used as a
device to measure fluid pressure in the plant. As he was
making his measurements of the root pressure of the grape
plants, he noted that the highest flow was measured in the
spring before the leaves came out, and that flow (measured
as a pressure) dropped off considerably later in the season
when the leaves were fully out. Hales then proceeded to
demonstrate that this seasonal reduction in flow, or pressure,
resulted from the evaporation of water from leaves that
pulled fluids from the roots up through the xylem. Hales
demonstrated this by removing leafy branches from a tree,
completely removing leaves from one branch and allowing
increasing amounts of leaves to be left on other branches.
Each branch then was set in a container with a known
volume of water. Over time, Hales observed that the
branches with the most leaves lowered the water level the
most in the containers.
Questions remained, however, about whether this move-
ment of fluids in plants was a closed-loop circulation, like
that in animals, or simply separate flows in separate tubes in
opposite directions. Hales devised an experiment to test the
circulation hypothesis in which he took a branch (from an
apple tree), attached a tube to the cut end of the branch, filled
the tube with water, and cut away the bark and the last
growth ring along 3 in. (inches) (7.6 cm [centimeters]) of
the branch just above the tube (Fig.
1.3
). He also made a cut
Fig. 1.3
A representation of Hales' experiment which determined that
plant fluid circulation is open, not closed like the circulation of blood in
mammals. Hales determined this when he attached a branch to a water-
filled tube and noted that water flow (
large arrow
) did not occur beyond
the first cut, and the upper cut remained dry. Hales also demonstrated
that if the leaves were immersed in a bucket of water, water moved in
the opposite direction, from leaves to the cut end.
in the bark about 1-ft (foot) (0.30 m [meters]) from the tube.
As he observed the water in the tube (which was 22 ft (6.7 m)
long) taken up by the branch, he noted that the first cut
became moister, whereas the upper cut remained dry.
Hence, he was able to prove that the movement of water in
a plant could not be described as a closed-loop circulation. In
fact, he showed that a branch could absorb water through
either end; sap, however, would only move from the shoots
to the roots. In 1727, Hales summed up his observations in
the publication
Vegetable Staticks
(or physics) (Hales 1969,
reprint).
1.1.7 Carolus Linnaeus, Georges-Louis Leclerc,
and the Organization of Plants
Increased international trade and immigration characterized
the sixteenth and seventeenth centuries and led to an
increase in the types of plants being observed by Europeans
and Middle Easterners. This increase in the number of plants
made it imperative for a systematic classification system to
be developed to handle the increasingly large body of known
plants. For instance, take the common potato; each area had
its own local name for this plant. The potato came to Spain
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