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

by Erasmus Darwin

Of aquatic leaves, see Note on Trapa and on Fucus, in Vol. II. to which must be added that many leaves are furnished with muscles about their footstalks, to turn their upper surfaces to the air or light, as Mimosa and 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 oxygene 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. Ingenhouze, some plants purified air less 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, that they converted the vital air into fixed air, or carbonic acid gas, in the same manner as in animal respiration. See Note XXXIV.

7. The circulation in the lungs or leaves of plants is very similar to that of fish. In fish the blood after having passed through their gills does not return to the heart as from the lungs of air-breathing animals, but the pulmonary vein taking the structure of an artery after having received the blood from the gills, which there gains a more florrid colour, distributes it to the other parts of their bodies. The same structure occurs in the livers of fish, whence we see in those animals two circulations independent of the power of the heart, viz. that beginning at the termination of the veins of the gills, and branching through the muscles; and that which passes through the liver; both which are carried on by the action of those respective arteries and veins. Monro's Physiology of Fish, p. 19.

The course of the fluids in the roots, leaves, and buds of vegetables seems to be performed in a manner similar to both these. First the absorbent vessels of the roots and surfaces unite at the footstalk of the leaf; and then, like the Vena Portarum, an artery commences without the intervention of a heart, and spreads the sap in its numerous ramifications on the upper surface of the leaf; here it changes its colour and properties, and becomes vegetable blood; and is again collected by a pulmonary vein on the under surface of the leaf. This vein, like that which receives the blood from the gills of fish, assumes the office and name of an artery, and branching again disperses the blood upward to the bud from the footstalk of the leaf, and downward to the roots; where it is all expended in the various secretions, the nourishment and growth of the plant, as fast as it is prepared.

II. The organ of respiration already spoken of belongs particularly to the shoots or buds, but there is another pulmonary system, perhaps totally independent of the green foliage, which belongs to the fructification only, I mean the corol or petals. In this there is an artery belonging to each petal, which conveys the vegetable blood to its extremities, exposing it to the light and air under a delicate membrane covering the internal surface of the petal, where it often changes its colour, as is beautifully seen in some party-coloured poppies; though it is probable some of the iridescent colours of flowers may be owing to the different degrees of tenuity of the exterior membrane of the leaf refracting the light like soap-bubbles, the vegetable blood is then returned by correspondent vegetable veins, exactly as in the green foliage; for the purposes of the important secretions of honey, wax, the finer essential oil, and the prolific dust of the anthers.

1. The vascular structure of the corol as above described, and which is visible to the naked eye, and its exposing the vegetable juices to the air and light during the day, evinces that it is a pulmonary organ.

2. As the glands which produce the prolific dust of the anthers, the honey, wax, and frequently some odoriferous essential oil, are generally attached to the corol, and always fall off and perish with it, it is evident that the blood is elaborated or oxygenated in this pulmonary system for the purpose of these important secretions.

3. Many flowers, as the Colchicum, and Hamamelis arise naked in autumn, no green leaves appearing till the ensuing spring; and many others put forth their flowers and complete their impregnation early in the spring before the green foliage appears, as Mezereon, cherries, pears, which shews that these corols are the lungs belonging to the fructification.

4. This organ does not seem to have been necessary for the defence of the stamens and pistils, since the calyx of many flowers, as Tragopogon, performs this office; and in many flowers these petals themselves are so tender as to require being shut up in the calyx during the night, for what other use then can such an apparatus of vessels be designed?

5. In the Helleborus-niger, Christmas-rose, after the seeds are grown to a certain size, the nectaries and stamens drop off, and the beautiful large white petals change their colour to a deep green, and gradually thus become a calyx inclosing and defending the ripening seeds, hence it would seem that the white vessels of the corol served the office of exposing the blood to the action of the air, for the purposes of separating or producing the honey, wax, and prolific dust, and when these were no longer wanted, that these vessels coalesced like the placental vessels of animals after their birth, and thus ceased to perform that office and lost at the same time their white colour. Why should they loose their white colour, unless they at the same time lost some other property besides that of defending the seed-vessel, which they still continue to defend?