Environmental Engineering Reference
In-Depth Information
magnify between 20 and 50 times, which pales in compari-
son to those that could magnify up to 300 times made in
1670 by Anton van Leeuwenhoek. Interestingly, when
Leeuwenhoek reported in 1678 that he had discovered little
animals with these microscopes, it was Hooke who was
tasked to confirm these findings.
Hooke's observations of cells in plant tissues were con-
firmed by other scientists and were seen in animal tissues as
well. Almost 200 year later, in 1838, the German scientists
Matthias Jakob Schleiden and Theodor Schwann linked
these observations of the presence of cells in both plants
and animals, respectively, and stated the Cell Theory of
Life—that all life consists of cells, and that all cells arise
from other cells.
Malpighi is regarded as the Father of Microanatomy. More-
over, unlike many contemporaries, such as Hooke and
Cesalpino, Malpighi did not think that fungi arose from
spontaneous generation from decaying organic matter, but
rather from seed or fragments of themselves.
Within this context of study and method of investigation,
it was Malpighi's causal observation of the jagged part of
bark that led him to study fluid flow in plants. In one of his
experiments, he removed the bark from completely around a
tree, a practice referred to as girdling. The bark contained the
tissues composed of cells that transport the food made in the
leaves, called phloem, but did not contain the tissues com-
posed of cells that transported water, called the xylem. His
experiment was designed to determine the role that these
structures have on fluid flow in plants, and whether or not it
was analogous to the closed-loop system in mammals. Over
time, he noted that the bark just above the area that had been
removed started to swell, and exuded a fluid, or sap, that was
sweet upon tasting. At first, the whole tree above the girdle
did not seem to suffer adversely from the experiment. Over a
period of a few weeks or months, however, Malpighi noted
that the leaves wilted and died, and death of the complete
tree soon followed. Malpighi concluded from his experiment
that water was transported up to the leaves from the soil
through the xylem, which after all had remained intact to the
tree after the bark had been removed. However, the tree died
not from a lack of water but because the roots had died due to
the lack of transport of the necessary sugars that were
formed in the leaves down to the roots, which cannot make
their own food.
Malpighi also designed an experiment in which he
observed a squash seedling planted in moist soil. He noticed
that the seedling stopped growing if its first leaf was
removed. This result suggested to him that the leaves were
important in some nutritive way to plants, much as Hooke
had hypothesized. Malpighi published the results of his
comparative anatomy of plants as
Anatome Plantarum
(Plant Anatomy) in 1675 (Kramer and Boyer 1995). Ironi-
cally, similar observations of the water-transporting
structures of wood were made again by Leeuwenhoek in
1676. Malpighi will surface again in Chap. 3 as the discov-
erer of another important part of plants, which turned out to
be directly related to the uptake of water by plants and,
therefore, useful
1.1.5 Marcello Malpighi and Fluid Flow
in Plants
Recognizing that water played an important role in the
growth of plants, scientists began to make attempts to answer
the question of
how
water was transported from soils through
plants and finally to the air. In the late 1600s, the Italian
scientist and physician Marcello Malpighi (1628-1694)
took this question to task. Although it may seem odd that a
physician would be interested in the flow of water in plants, at
that time there was great interest in the study of the hidden
flow of blood in the circulatory system of mammals. This
inquiry had been initiated by the hypotheses of William
Harvey, who realized in 1628, that the human heart was a
pump, not a source of heat as had been envisioned up until
this time. Harvey stated that this pump pushed blood through
veins, which had always been known to contain blood, as
well as through arteries, which had long been thought to
either be empty or a redundant source of blood. It was Harvey
that hypothesized that the two vessels were connected in a
closed loop that passed through the heart.
In any case, Harvey's hypothesis was missing a major
part; he had not been able to see what connected, or closed,
the loop between the arteries and the veins. In 1661,
Malpighi used microscopes to investigate blood flow in
frogs and observed the presence of the smallest blood
vessels, or capillaries, in the lungs. These observations
provided the experimental evidence needed to demonstrate
that capillaries connected the flow of blood in the arteries
with that of the veins. Although Leeuwenhoek had seen
capillaries in the tails of fish placed under his microscope
earlier than Malpighi, for whatever reason Leeuwenhoek's
observations are often forgotten. Malpighi extended such a
microscopic approach to the comparative anatomy of vari-
ous organs in the body of mammals, including humans. He
also discovered that insects breathed not with lungs, but
through a row of holes on their bodies. For all his efforts,
to phytoremediation of contaminated
groundwater.
Another scientist, the medical doctor Nehemiah Grew
(1641-1712), also was interested in the fluid flow of plants.
He speculated that the circulation of fluids in plants may be
similar to that of the circulation of blood in humans. Some of
his anatomical observations were later published in
The
Anatomy of Plants
(1682) (Kramer and Boyer 1995). Grew
also advanced the idea that roots act as 'mouths' that ingest
water from the soil along with air.
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