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. 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.