Environmental Engineering Reference
In-Depth Information
therefore, used water to support their sustenance, rather than
soil.
Van Helmont reached this conclusion, however, without
the benefit of having recorded the total amount of water
added over the 5-year period. In fact, neither Aristotle nor
van Helmont was entirely correct; plants use both water and
substances from the soil. What they could not have realized
at the time, however, was the role that air, which contained
gases such as carbon dioxide (CO 2 ), played in plant growth
and in the increase in the willow's weight. It was not until
recent times that it was revealed that plants in fact do take in
substances, both mineral and organic, from dead or decaying
organic matter in the soil. Aside from the rather dubious
connection to alchemy and lack of meticulous quantification,
the process of turning water into plant matter provided the
first evidence that plants could take up significant quantities
of water from soils. Moreover, van Helmont's experiments
confirmed similar speculation made earlier by Sir Francis
Bacon (1561-1626) that plants used water.
A similar conclusion regarding the importance of water
for plant growth was made in 1700 by John Woodward
(1665-1728). Woodward grew plants in pots much like did
van Helmont, but only in water obtained from various
sources, such as rainwater, or river water. Woodward
observed the best growth was achieved when soils were
added to the water. Additional confirmation that water and
soil were important to plant growth was provided by the
experiments of Jethro Tull (1674-1741). Tull stated that
plants were sustained on fine particles of soil that entered
the plant through the roots. This idea gave rise to the then
novel but now common practice of tilling, the goal of which
is to produce very fine soil particles for nutrient release.
that he called cella (from the Latin word for cell , or small
room) (Fig. 1.2 ). He wrote
...
I could exceedingly plainly perceive it to be all perforated
and porous, much like a honey-comb, but that the pores of it
were not regular
...
these pores, or cells
...
were indeed the first
microscopical pores I ever saw
...
Observation XVIII, Micrographia (Hooke 1665)
This microscopic observation of cork cells helped to
explain the characteristics of cork perceived at the macro-
scopic scale: a light-weight, nonabsorbent, compressible
material. Hooke stated that this behavior could be explained
by the fact that cork was actually a very small quantity of a
solid but was spread out over a large dimension. He also
observed a similar structure in elder tree pith, the tissues at
the center of stems composed primarily of what turned out to
be parenchymal cells. It was fortunate that Hooke's micro-
scope was powerful enough to resolve individual dead cork
cells, because cork cells are relatively thin and light in
weight.
These observations of cellular spaces in non-living
tissues led Hooke to speculate not just on its effect on plant
structure but also the novel role of air in plant growth. In
1665, Hooke performed a simple experiment that proved
that plants need to be exposed to air to survive. He placed
seeds in soil both covered and uncovered with a glass jar; the
covered seeds did not germinate. Hooke published these and
other observations, drawn by hand, in 1665 in Micrographia ,
which became a best seller and was widely read even
by non-scientists. Hooke's compound microscopes could
1.1.4 Robert Hooke and Cells
As has been the case throughout history, rapid advances in
the sciences usually follow the introduction of a new tool,
piece of analytical equipment, or a difference in experimen-
tal approach. For example, in the early seventeenth century,
many technicians were grinding lenses to make compound
microscopes in an attempt to observe physical processes at
scales of ever increasing resolution. This had not been done
before. All that had been studied was the appearance of
everyday things.
One of the first observations made with a microscope
was related to plants, and was made by Robert Hooke
(1635-1703). Hooke was an innovative scientist who made
major discoveries in many fields. Using a two-lensed micro-
scope he made himself, Hooke observed that the seemingly
solid structure of dead cork tissues from the pith of an alder
tree was, in fact, not a homogenous solid mass but rather
composed of multiple copies of smaller, empty structures
Fig. 1.2 A representation of an etching of cork cells from an alder tree
similar to that drawn by Robert Hooke in his Micrographia (Hooke
1665). As most of this tissue is composed of air space, his observations
explained why cork was lighter in weight than most woody plant
samples of similar volume.
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