A History of Science, vol 4 by Henry Smith Williams (best novels to read for beginners txt) 📖
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substances, especially of the vegetable and animal kingdoms; and
although this menstruum is capable of acting independently of the
stomach, yet it is indebted to that viscus for its
continuance.[5]
THE FUNCTION OF RESPIRATIONIt is a curious commentary on the crude notions of mechanics of
previous generations that it should have been necessary to prove
by experiment that the thin, almost membranous stomach of a
mammal has not the power to pulverize, by mere attrition, the
foods that are taken into it. However, the proof was now for the
first time forthcoming, and the question of the general character
of the function of digestion was forever set at rest. Almost
simultaneously with this great advance, corresponding progress
was made in an allied field: the mysteries of respiration were
at last cleared up, thanks to the new knowledge of chemistry. The
solution of the problem followed almost as a matter of course
upon the advances of that science in the latter part of the
century. Hitherto no one since Mayow, of the previous century,
whose flash of insight had been strangely overlooked and
forgotten, had even vaguely surmised the true function of the
lungs. The great Boerhaave had supposed that respiration is
chiefly important as an aid to the circulation of the blood; his
great pupil, Haller, had believed to the day of his death in 1777
that the main purpose of the function is to form the voice. No
genius could hope to fathom the mystery of the lungs so long as
air was supposed to be a simple element, serving a mere
mechanical purpose in the economy of the earth.
But the discovery of oxygen gave the clew, and very soon all the
chemists were testing the air that came from the lungs—Dr.
Priestley, as usual, being in the van. His initial experiments
were made in 1777, and from the outset the problem was as good as
solved. Other experimenters confirmed his results in all their
essentials—notably Scheele and Lavoisier and Spallanzani and
Davy. It was clearly established that there is chemical action
in the contact of the air with the tissue of the lungs; that some
of the oxygen of the air disappears, and that carbonic-acid gas
is added to the inspired air. It was shown, too, that the blood,
having come in contact with the air, is changed from black to red
in color. These essentials were not in dispute from the first.
But as to just what chemical changes caused these results was the
subject of controversy. Whether, for example, oxygen is actually
absorbed into the blood, or whether it merely unites with carbon
given off from the blood, was long in dispute.
Each of the main disputants was biased by his own particular
views as to the moot points of chemistry. Lavoisier, for
example, believed oxygen gas to be composed of a metal oxygen
combined with the alleged element heat; Dr. Priestley thought it
a compound of positive electricity and phlogiston; and Humphry
Davy, when he entered the lists a little later, supposed it to be
a compound of oxygen and light. Such mistaken notions naturally
complicated matters and delayed a complete understanding of the
chemical processes of respiration. It was some time, too, before
the idea gained acceptance that the most important chemical
changes do not occur in the lungs themselves, but in the ultimate
tissues. Indeed, the matter was not clearly settled at the close
of the century. Nevertheless, the problem of respiration had
been solved in its essentials. Moreover, the vastly important
fact had been established that a process essentially identical
with respiration is necessary to the existence not only of all
creatures supplied with lungs, but to fishes, insects, and even
vegetables—in short, to every kind of living organism.
ERASMUS DARWIN AND VEGETABLE PHYSIOLOGYSome interesting experiments regarding vegetable respiration were
made just at the close of the century by Erasmus Darwin, and
recorded in his Botanic Garden as a foot-note to the verse:
“While spread in air the leaves respiring play.”
These notes are worth quoting at some length, as they give a
clear idea of the physiological doctrines of the time (1799),
while taking advance ground as to the specific matter in
question:
“There have been various opinions,” Darwin says, “concerning the
use of the leaves of plants in the vegetable economy. Some have
contended that they are perspiratory organs. This does not seem
probable from an experiment of Dr. Hales, Vegetable Statics, p.
30. He, found, by cutting off branches of trees with apples on
them and taking off the leaves, that an apple exhaled about as
much as two leaves the surfaces of which were nearly equal to the
apple; whence it would appear that apples have as good a claim to
be termed perspiratory organs as leaves. Others have believed
them excretory organs of excrementitious juices, but as the vapor
exhaled from vegetables has no taste, this idea is no more
probable than the other; add to this that in most weathers they
do not appear to perspire or exhale at all.
“The internal surface of the lungs or air-vessels in men is said
to be equal to the external surface of the whole body, or almost
fifteen square feet; on this surface the blood is exposed to the
influence of the respired air through the medium, however, of a
thin pellicle; by this exposure to the air it has its color
changed from deep red to bright scarlet, and acquires something
so necessary to the existence of life that we can live scarcely a
minute without this wonderful process.
“The analogy between the leaves of plants and the lungs or gills
of animals seems to embrace so many circumstances that we can
scarcely withhold our consent to their performing similar
offices.
“1. The great surface of leaves compared to that of the trunk
and branches of trees is such that it would seem to be an organ
well adapted for the purpose of exposing the vegetable juices to
the influence of the air; this, however, we shall see afterwards
is probably performed only by their upper surfaces, yet even in
this case the surface of the leaves in general bear a greater
proportion to the surface of the tree than the lungs of animals
to their external surfaces.
“2. In the lung of animals the blood, after having been exposed
to the air in the extremities of the pulmonary artery, is changed
in color from deep red to bright scarlet, and certainly in some
of its essential properties it is then collected by the pulmonary
vein and returned to the heart. To show a similarity of
circumstances in the leaves of plants, the following experiment
was made, June 24, 1781. A stalk with leaves and seed-vessels of
large spurge (Euphorbia helioscopia) had been several days placed
in a decoction of madder (Rubia tinctorum) so that the lower part
of the stem and two of the undermost leaves were immersed in it.
After having washed the immersed leaves in clear water I could
readily discover the color of the madder passing along the middle
rib of each leaf. The red artery was beautifully visible on the
under and on the upper surface of the leaf; but on the upper side
many red branches were seen going from it to the extremities of
the leaf, which on the other side were not visible except by
looking through it against the light. On this under side a system
of branching vessels carrying a pale milky fluid were seen coming
from the extremities of the leaf, and covering the whole under
side of it, and joining two large veins, one on each side of the
red artery in the middle rib of the leaf, and along with it
descending to the foot-stalk or petiole. On slitting one of these
leaves with scissors, and having a magnifying-glass ready, the
milky blood was seen oozing out of the returning veins on each
side of the red artery in the middle rib, but none of the red
fluid from the artery.
“All these appearances were more easily seen in a leaf of Picris
treated in the same manner; for in this milky plant the stems and
middle rib of the leaves are sometimes naturally colored reddish,
and hence the color of the madder seemed to pass farther into the
ramifications of their leaf-arteries, and was there beautifully
visible with the returning branches of milky veins on each side.”
Darwin now goes on to draw an incorrect inference from his
observations:
“3. From these experiments,” he says, “the upper surface of the
leaf appeared to be the immediate organ of respiration, because
the colored fluid was carried to the extremities of the leaf by
vessels most conspicuous on the upper surface, and there changed
into a milky fluid, which is the blood of the plant, and then
returned by concomitant veins on the under surface, which were
seen to ooze when divided with scissors, and which, in Picris,
particularly, render the under surface of the leaves greatly
whiter than the upper one.”
But in point of fact, as studies of a later generation were to
show, it is the under surface of the leaf that is most abundantly
provided with stomata, or “breathing-pores.” From the standpoint
of this later knowledge, it is of interest to follow our author a
little farther, to illustrate yet more fully the possibility of
combining correct observations with a faulty inference.
“4. As the upper surface of leaves constitutes the organ of
respiration, on which the sap is exposed in the termination of
arteries beneath a thin pellicle to the action of the atmosphere,
these surfaces in many plants strongly repel moisture, as cabbage
leaves, whence the particles of rain lying over their surfaces
without touching them, as observed by Mr. Melville (Essays
Literary and Philosophical: Edinburgh), have the appearance of
globules of quicksilver. And hence leaves with the upper
surfaces on water wither as soon as in the dry air, but continue
green for many days if placed with the under surface on water, as
appears in the experiments of Monsieur Bonnet (Usage des
Feuilles). Hence some aquatic plants, as the water-lily
(Nymphoea), have the lower sides floating on the water, while the
upper surfaces remain dry in the air.
“5. As those insects which have many spiracula, or breathing
apertures, as wasps and flies, are immediately suffocated by
pouring oil upon them, I carefully covered with oil the surfaces
of several leaves of phlomis, of Portugal laurel, and balsams,
and though it would not regularly adhere, I found them all die in
a day or two.
“It must be added that many leaves are furnished with muscles
about their foot-stalks, to turn their surfaces to the air or
light, as mimosa or Hedysarum gyrans. From all these analogies I
think there can be no doubt but that leaves of trees are their
lungs, giving out a phlogistic material to the atmosphere, and
absorbing oxygen, or vital air.
“6. The great use of light to vegetation would appear from this
theory to be by disengaging vital air from the water which they
perspire, and thence to facilitate its union with their blood
exposed beneath the thin surface of their leaves; since when pure
air is thus applied it is probable that it can be more readily
absorbed. Hence, in the curious experiments of Dr. Priestley and
Mr. Ingenhouz, some plants purified less air than others—that
is, they perspired less in the sunshine; and Mr. Scheele found
that by putting peas into water which about half covered them
they converted the vital air into fixed air, or carbonic-acid
gas, in the same manner as in animal respiration.
“7. The circulation in the lungs or leaves of plants is very
similar to that of fish. In
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