A History of Science, vol 4 by Henry Smith Williams (the two towers ebook .TXT) 📕

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]

It 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.

Some 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