‘While the leaves owe their first nourishment principally to the more or less modified watery parts, which they draw from the stem, they are indebted for their increased perfection and refinement to the light and air. The cotyledons which are formed beneath the closed seed-sheath are charged, so to speak, with only a crude sap; they are scarcely and but rudely organized and quite undeveloped. In the same way the leaves are more rudely organized in plants which grow under water than in others which are exposed to the open air. Indeed, even the same species of plant develops smoother and less intricately formed leaves when growing in low damp places, whereas, if transplanted to a higher region, it will produce leaves which are rough, hairy and more delicately finished.’