STEPHEN HALES
The Circulation of Sap in
Plants
Stephen Hales was born to a well-to-do
family in Bekesbourne, Kent, in 1677. In 1696 he entered Bene't College,
Cambridge. At that time the educational opportunities at Cambridge were
remarkably diverse. With his friend William Stukeley he seems to have combined
extensive studies in natural history and biology with a great interest in the
physics of fluids, gases and liquids. So from the very earliest knowledge we
have of him, the Leitmotif of his scientific and engineering work was apparent,
the role of pneumatic and fluid dynamics in the processes of life.
Hales remained in Cambridge as a Fellow of his
College until 1709, when he became the Vicar of Teddington, a post he held for
the rest of his life. Though Harvey is credited with the `discovery' of the
circulation of the blood in men and animals, this amounted to no more than a
theoretical demonstration of the necessity of such a hypothesis given the facts
about how much blood the body contained. In a long series of both gruesome and
rigorous experiments on horses, dogs and frogs, Hales explored many aspects of
the blood vascular system, charting its pathways and exploring the hydrodynamic
conditions of pressure and flow that characterized each part. His work was
definitive, solving many of the major problems left by Harvey's inspired
hypothesis.*
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*A broad concept of blood pressure, blood flow, blood velocity and
their relations, and quantitative measurements or calculations of each -
these were the great contributions of Stephen Hales to the knowledge of the
output of the heart, a contribution which has oriented all future work.' W, F.
Hamilton and D. W. Richards in Circulation of the Blood: Men and Ideas, edited
by A, D. Fishman and D. W. Richards, New York, 1964.
--------
But
these investigations did not pass unnoticed by the general public. Thomas
Twining includes a verse in his topographical poem The Boat that runs as
follows:
Green Teddington's serene retreat
For philosophic studies meet,
Where the good Pastor Stephen Hales
Weighed moisture in a pair of scales,
To lingering death put Mares and Dogs,
And stripped the Skins from living Frogs.
Nature, he loved, her Works intent
To search, and sometimes to torment.
Though the movement against thoughtless cruelty to
animals had begun about this time, its leading protagonist, Alexander Pope, a
neighbour of Hales, became one of his closest friends.
In about 1724 Hales began the series of studies that
established the main outlines of the physiology of plants. Not only did he
study the way the sap circulated, but most importantly the interactions and
exchanges between the plant and its environment. He showed how the water drawn
in by the roots is transported to the leaves and there transpired. Growth, too,
interested him, and he demonstrated how the various parts of plants grow and in
what proportions. Mayow had shown the relation between respiration, combustion
and the air some years before Hales began his studies, and he pursued this
problem too.
In 1722 Hales was elected a Fellow of the Royal
Society, becoming a member of the Council in 1727. He had become a public
figure of considerable eminence, a Trustee for the Colony of Georgia, and a
regular member of commissions appointed to look into matters of public health,
such as conditions in the ships of the Royal Navy, and the examination of
alleged wonder cures. His interest in air extended to ventilation. The problem
of getting fresh air into confined spaces such as the living quarters of ships,
prisons and hospitals became for a while his chief preoccupation. He invented a
variety of ventilating devices, most of which were put into practical use. He
died in 1761, still the Vicar, of Teddington.
Early work on the hydrodynamics of plants
Botanical studies in antiquity were dominated by the
work of Theophrastus, a pupil of Aristotle. Most works were descriptive and
classificatory, grouping plants by reference to their general form, such as
herbs, bushes and trees, or by their alleged medicinal properties. These
classifications came through into the Middle Ages. They were thoroughly
practical in intent, if greatly corrupted in substance after innumerable and
inaccurate copyings. Theophrastus had also made some study of the relation of
plants to their typical environments, cross-classifying by reference to
habitat. But this aspect of his work had degenerate& into little more than
a guide for where to look for specific herbal remedies. So far as we know there
were no anatomical or physiological studies of plants in antiquity.
The first substantial modern
work was made possible by the development of the microscope in the mid-seventeenth
century. Robert Hooke, the same who had served as Boyle's assistant, made
careful microscopical examinations of plants. He was the first to identify the
cell as the basic biological unit. Nehemiah Grew carried this kind of work very
much farther, making detailed studies of the anatomy of plants, and producing
anatomical drawings of the highest quality.
The most important discovery
to come out of the use of the microscope was the realization that the plant
contained ramifying systems of tubes, running from the roots through the stem
and branches to the leaves. Some of the tubes seemed to be filled with liquid,
others with air. Considering this system Grew came to realize the possibility
of a circulation in plants comparable to that known to occur in animals. Once
this thought was formulated all sorts of questions sprang to mind. Was there a
closed circulation in plants as there was in animals? What force powered the
flow of sap? Relative to this circulation what were the life functions of the
various parts of the plant? It was to these questions that Hales devoted his
great experimental series.
The circulation of the sap
The basic theory of the vital processes of plants
had been formulated about 1670 by Malpighi. He had grasped two points of
crucial importance. Common sense had suggested that there must be a movement of
sap upwards from the roots towards the leaves, contributing at least the watery
element to the whole plant. Malpighi realized that the elaboration of simpler
elements into plant substance took place in the leaves.
It followed that there must also be a downward
movement to carry body-building substances from the leaves to the other
parts of the plant where they were to be used. He also understood the process
that led to the production and storage of a surplus for later use. Since in
many plants this material was stored in tubers associated with the roots, the
counter circulation of nutriments must reach as far as the roots, the very
source of the primary circulation of water. All this was informed speculation.
It remained to be demonstrated experimentally. This was Hales's contribution.
As in so much scientific
work the central experiment which I shall describe was the culmination of a
series of subsidiary experiments preparing the way for it. First it was
necessary to determine whether the throughput of water from roots to leaves was
a process powered by pressure from the roots or by some drawing process from
the leaves.
'July 27 [1716]. I fixed an
Apple-branch . . . to a tube. I filled the tube with water, and then
immersed the whole branch . . . into the vessel u u full of water.'
'The water subsided 6 inches
the first two hours (being the filling of the sap vessels) and 6 inches the
following night . . . . The third day in the morning, I took the branch out of
the water; and hung it with the Tube affixed to it in the open air; it imbibed
this 27 + 1/z inches in 12 hours.' Hales concluded that this experiment 'shews
the great power of perspiration'. It is the evaporation of water from the
leaves, not the pressure of water in the roots that is the prime mover in the
circulation of the sap. Of course these experiments do not show how these
processes come about.
But is it water that is
transpired from the leaves? That the fluid is mostly water can be demonstrated
neatly by confining a leafy branch in a vessel and collecting the 'perspired'
fluid.
Now the stage was set for
the key experiment: how does the sap move? Is it a circulation as the animal
analogy; would suggest, or is it a kind of tidal ebb and flow? In two perfect
experiments Hales cleared this matter up for all time. The circulationists had
assumed that the sap moved up in the inner part of the stem and down in the
outer.
On August 20 [ 1716] he
says, 'at 1 p.m. I took an Applebranch b nine feet long, 1 + 3/4 inch
diameter, with proportional lateral branches, I cemented it fast to the tube
a, by means of the lead syphon 1; but first I cut away the bark, and last
year's ringlet of wood, for 3 inches length to r. I then filled the tube with
water, which was 22 feet long and 1/2 inch diameter, having first cut a gap at
y through the bark, and last year's wood 12 inches from the lower end of the
stem: the water was very freely imbibed, viz. at the rate of 3 + 1/2 inch in a
minute. In half an hour's time I could plainly perceive the lower part of the
gap y to be moister than before; when at the same time the upper part of the
wound looked white and dry.'
It follows that `the water must necessarily ascend
from the tube, through the innermost wood, because the last year's wood was cut
away, for 3 inches length all round the stem; and consequently, if the sap in
its natural course descended by the last year's ringlet of wood, and between
that and the bark (as many have thought) the water should have descended by the
last year's wood, or the bark, and so have first moistened the upper part of
the gap y; but on the contrary, the lower part was moistened, and not the upper
part.' Since the sap must be ascending by the inner part of the stem, there
being a ring cut right round below the gap y, and since it is also ascending by
the last year's wood and the bark, as evidenced by the moisture forming at
lower side of the gap, there is no circulation, at least not in the strict
sense of a complete hydraulic cycle. If there had been a cycle, movement in one
direction in one part would have been compensated for by correlative movement ,
in another direction, somewhere else in the system.
Further strong indirect evidence can be found for
this conclusion, from a consideration of how much water a plant takes up and
transpires in a day. Hales showed that the sunflower, transpires water at a
rate seventeen times that of a man, bulk for bulk. If there were a circulation
it would have to be enormously fast. But there is no evidence whatsoever for
such celerity of movement.
But the
sap does in some measure recede from the top
to the bottom of plants', as many ingenious experiments have proved, so Hales
notes. But this does not demonstrate a circulation, rather a daily ebb and
flow.
Developments in plant physiology after Hales
It is quite fair to say that in the hundred years immediately
following the masterly series of experiments of which I have described only one
particularly ingenious fragment, Hales's successors added little to the science
of plant physiology.
Fig.
11. In this plate from the Vegetable Staticks (1738). Fig. 26 illustrates the
third experiment. The notches used to test the 'circulation' hypothesis can be
seen at y and q.
However,
some contributions were made in this period. Hales's experiments had almost
fully clarified the water economy of plants. But plants are also exchanging
gases with the atmosphere. Mayow (the first scientist clearly to distinguish
the gases of the atmosphere) and Hales had both suspected that plants took some
of their nourishment from the air. Hales had distinguished the kinds of gaseous
exchange, the nutritive and the respiratory. But he had failed to understand
Mayow's discovery of a constituent of air, 'spiritus nitro-aereus' (or
`oxygen' as we now call it), which was absorbed or 'fixed' in vital processes.
Hales had supposed that respiration and combustion reduced the volume of air by
one fifth because the air had lost that proportion of its elasticity, rather
than that one fifth of its substance had been absorbed. Given this quite
central error in his theory of the air Hales was unable clearly to identify the
nutritive and respiratory gaseous exchanges for what they were. In 1779, the
Dutch doctor Ingenhousz established that there were two quite distinct
respiratory cycles in the life of plants. In one cycle oxygen was absorbed and
carbon dioxide exhaled just as in animal respiration. In the other cycle carbon
dioxide was taken in as a kind of gaseous food, and oxygen was given out. By
about 1840 the chemistry of the gases of the air was well known. Oxygen,
nitrogen and carbon dioxide had been clearly distinguished and their chemical
properties thoroughly investigated. The final step came in 1840 when
Boussingault showed that plants obtained their nitrogen not from the air, but
from the nitrates present in the soil in which they grew.
Further
reading
Hales, S.,
Vegetable Staticks, London, 1727. A fine modern reprint has been edited by M.
A. Hoskin, Oldbourne Science Library, London, 1961.
Allan, D. G.
C., and Schofield, R. E., Stephen Hales: Scientist and Philanthropist, London,
1980.
Clark-Kennedy,
A. E., Stephen Hales, D.D., F.R.S.: An Eighteenth Century Biography, Cambridge-New
York, 1929; repr. Ridgewood, N.J., 1965.
von Sachs,
J., History of Botany, transl. H. E. F. Garnsey and I. B. Balfour, Oxford,
1906.