The Botanic Garden, a Poem in Two Parts. Part 1: the Economy of Vegetation

by Erasmus Darwin

CANTO IV. l. 421.

I. There have been various opinions concerning the use of the leaves of plants in the vegetable oeconomy. Some have contended that they are perspiratory organs; this does not seem probable from an experiment of Dr. Hales, Veg. Stat. 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 excrementious juices; but as the vapour exhaled from vegetables has no taste, this idea is no more probable than the other; add to this that in moist weather, they do not appear to perspire or exhale at all.

The internal surface of the lungs or air-vessels in men, are said to be equal to the external surface of the whole body, or about 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 colour 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 assent to their performing similar offices.

I. The great surface of the 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 lungs of animal, the blood after having been exposed to the air in the extremities of pulmonary artery, is changed in colour 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 shew a similarity of circumstance 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 discern the colour of the madder passing along the middle rib of each leaf. This red artery was beautifully visible both on the under and 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 underside of it, and joining into 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 footstalk or petiole. On slitting one of these leaves with scissars, and having a common magnifying lens 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 coloured reddish, and hence the colour of the madder seemed to pass further into the ramifications of their leaf-arteries, and was there beautifully visible with the returning branches of milky veins on each side.

3. From these experiments the upper surface of the leaf appeared to be the immediate organ of respiration, because the coloured 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 scissars, and which in Picris, particularly render the under surface of the leaves greatly whiter than the upper one.

4. As the upper surface of leaves constitutes the organ of respiration, on which the sap is exposed in the terminations 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 Philosop. Edinburgh) have the appearance of globules of quicksilver. And hence leaves laid with the upper surfaces on water, wither as soon as in the dry air, but continue green many days, if placed with the under surfaces on water, as appears in the experiments of Mons. Bonnet (Usage des Fevilles.) Hence some aquatic plants, as the Water-lily (Nymphoea) have the lower sides of their leaves 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.