Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy.

  • Davy, Humphry, Sir, 1778-1829.
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy. Source: Wellcome Collection.

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    fibres will twist so as to bring it as much as possible into its original position; and all leaves elevate themselves on the foot-stalk during their exposure to the solar light, and as it were move towards the sun. This effect seems in a great measure dependent upon the mechanical and chemical agency of light and heat. Bonnet made artificial leaves, which, when a moist sponge was held under the lower surface, and a heated iron above the upper surface, turned exactly in the same manner as the natural leaves. This, however, can be considered only as a very rude imitation of the natural process, (see note, page 18.) What Linnaeus has called the sleep of the leaves, appears to depend wholly upon the defect of the action of light and heat, and the excess of the operation of moisture. This singular but constant phenomenon had never been scientifically observed, till the attention of the botanist of Upsal was fortunately directed to it. He was examining particularly a species of lotus, in which four flowers had appeared during the day, and he missed two in the evening; by accurate inspection, he soon discovered that these two were hidden by the leaves which had closed round them. Such a circumstance could not be lost upon so acute an observer. He immediately took a lantern, went into his garden, and witnessed a series of curious facts before unknown. All the simple leaves of the plants he examined, had an arrangement totally different from their arrangement in the day: and the greater number of them were seen closed or folded together. The sleep of leaves is, in some cases, capable of being produced artifi- cially. Decandolle made this experiment on the sensitive plant. By confining it in a dark place in the day time, the leaves soon closed; but on illuminating the chamber with many lamps, they again expanded. So sensible were they to the effects of light and radiant heat. In the greater number of plants the leaves annually decay, and are reproduced; their decay takes place either at the conclusion of the sum- mer, as in very hot climates, when they are no longer supplied with sap, in consequence of the dryness of the soil, and the evaporating powers of heat; or in the autumn, as in the northern climates, at the commencement of the frosts. The leaves preserve their functions, in common cases, no longer than there is a circulation of fluids through them. In the decay of the leaf, the colour assumed seems to depend upon the nature of the chemical change, and as acids are generally developed, it is usually either reddish brown or yellow; yet there are great varieties. Thus, in the oak it is bright brown; in the beech, orange; in the elm, yellow; in the vine, red; in the sycamore, dark brown; in the cornel tree, purple; and in the woodbine, blue.*' The cause of the preservation of the leaves of evergreens through the winter is not accurately known. From the experiments of Hales, it appears that the force of the sap is much less in plants of this species, and probably there is a certain degree of circulation throughout the winter; * The variety in the tints of green of our finest trees has often been^ remarked, but the richer and more striking variety of their autumnal foliage has been less noticed, partly on account of its short duration, and partly because it is only when still and serene weather occurs in autumn, that the leaves continue to hang in sufficient numbers to afford a remarkable contrast between the tints of different species. Artists are well aware of the circumstance, and some of them have availed themselves of this knowledge in a remarkable manner. The fall of the leaf is ascribed in the text to drought, in warm climates, and to frosts in colder ones. These causes no doubt concur with another, the filling