Prillieux, Roze, and Brogniart, who find that the rotation is directly-influenced, in a remarkable manner, by the presence of light. M. Prillieux kept a moss in the dark for several days, when the cells presented the appearance of a green network, between the meshes of which was a clear, transparent ground. All the grains of chlorophyll were applied to the walls which separate the cells from one another; there were none on the upper or under walls which form the surfaces of the leaf. Under the influence of light, the grains, together with the thin mucous plasma in which they are embedded, change their position from the lateral to the superficial walls, this change taking place, under favorable circumstances, in about a quarter of an hour. On attaining their new position, the grains do not remain absolutely immovable, but continually approach and recede from one another; and, if again darkened, they leave their new position, and return to the lateral walls. Artificial light produces the same effect as daylight.
Analogous to the circulation of the protoplasm, within the cell, is that of the sap or nutritive fluid through the whole plant, passing through the permeable walls of the cells. This circulation of the sap, by which fluid is conveyed equally to all parts of the plant, apparently in opposition to the laws of gravity, is no doubt explicable, to a certain extent, by the application of known physical laws, of which the most important are capillary attraction, osmose, or the law by which a less dense fluid passes through a permeable diaphragm to mingle with a denser fluid, and the upward-pumping force, to supply the partial vacuum occasioned by the evaporation of water from the leaves. Allowing, however, full scope to all these physical forces, there would seem to be a residuum of energy, still unaccounted for, connected with the vitality of the plant itself. In particular, the selective power of plants, in absorbing from the soil a larger portion of those ingredients which are required for the formation, or healthy life, of their tissues, is an absolutely unexplained phenomenon. A familiar instance of this is furnished by the difference in the amount of silica absorbed by corn-crops and by leguminous plants, amounting, in the former case, to 2.5 per cent., in the latter, to 0.3 per cent., of the dry foliage. Indeed, if any two plants are grown together, side by side, in the same soil, the constitution of the ash, i. e., of the solid ingredients derived from the soil, will be remarkably different; while, in the same plant, in the same soil, the constitution is constant. It was pointed out, by the Duke of Argyll, when criticising Darwin's "Origin of Species," how unavoidable it seems, in describing the phenomena of Nature, to use language involving the idea of contrivance and design. In the same manner, it seems impossible to describe the process of vegetative life without appearing to attribute to the plant some conscious power of its own. A striking instance of this, as well as of the liability to consider a mere statement of an obscure law in other terms as an explanation of that law, occurs in an admirable treatise on the growth of plants—Johnson's
