2 84 THE POPULAR SCIENCE MONTHLY. 



Prillieux, Roze, and Brogniart, who find that the rotation is directly- 

 influenced, in a remarkable manner, by the presence of light. M. Pril- 

 lieux kept a moss in the dark for several days, when the cells pre- 

 sented the appearance of a green net-work, 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 posi- 

 tion from the lateral to the superficial walls, this change taking place, 

 under favorable circumstances, in about a quarter of an hour. On at- 

 taining their new position, the grains do not remain absolutely immov- 

 able, 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 cer- 

 tain 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. Al- 

 lowing, 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 unavoid- 

 able 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 



