Page:Proceedings of the Royal Society of London Vol 4.djvu/457

Rh objections are not valid, and to prove, by a series of experiments, that the motion of the sap is totally independent of any vital con- tractions of the passages which transmit it ; that it is wholly a mecha- nical process, resulting entirely from the operation of endosmose ; and that it takes place even through those parts of a plant which have been totally deprived of their vitality.

The lower extremity of a branch of Valeriana rubra was placed, soon after being gathered, into a solution of bichloride of mercury. In a few hours a considerable quantity of this solution was absorbed, and the whole plant, which had been previously somewhat shrunk from the evaporation of its moisture, recovered its healthy appear- ance. On the next day, although the lower portion of the branch had lost its vitality, the leaves and all the parts of the plant into which no bichloride had entered, but only the water of the solution, were perfectly healthy and filled with sap. On each of the following days additional portions of the stem became affected in succession ; but the unaffected parts still preserved their healthy appearance, and the flowers and leaves developed themselves as if the plant had ve- getated in pure water and the whole stem had been in its natural healthy state. On a minute examination it was found that calomel, in the form of a white substance, had been deposited on the internal surface of the cuticle ; but no bichloride of mercury could be de- tected in those parts which had retained their vitality ; thus showing that the solution of the bichloride had been decomposed into chlo- rine, calomel, and water, and had destroyed the vitality of the parts where this action had taken place ; after which, fresh portions of the solution had passed through the substance of the poisoned parts, as if they had been inorganic canals. Various experiments of a similar kind were made on other plants, and the same conclusions were de- duced from them.

As the addition of a solution of iodide of potassium converts the bichloride of mercury into an insoluble biniodide, the author was enabled, by the application of this test to thin sections of the stems of plants into which the bichloride had been received by absorption, to ascertain, with the aid of the microscope, the particular portion of the structure into which the latter had penetrated. The result of his observations was, that the biniodide is found only in the in- tercellular and intervascular spaces, none appearing to be contained within the cavities of either cells or vessels.

As the fluids contained in the vessels and in the cells hold in solution various vegetable compounds, their density is greater than the ascending sap, which is external to them, and from which they are separated by an intervening organized membrane. Such being the conditions requisite for the operation of the principle of endosmose, the author infers that such a principle is constantly in action in living plants ; and that it is the cause of the continual transmission of fluids from the intervascular and intercellular spaces into the interior of the vessels and cells, and also of the ascent of the sap.