ARE LIVING CELLS INVOLVED IN THE ASCENT OF SAP?

Abstract

IN A RECENT PAPER (Peirce, 1934) I called attention to certain features of the problem of the ascent of sap through the stems of land plants, and I set forth a hypothesis of sap hydraulics which, in certain respects, differs from the currently accepted Dixon (1914) theory. As Dixon. living in the humid atmosphere of Ireland, was influenced by his environment in forming his opinions, so no doubt my ideas are among the products of many years of residence in a semi-arid climate. In my recent paper I left many questions unanswered, many statements to be confirmed or corrected. On this occasion I wish to speak of one. In 1884 the senior Godlewski definitely expressed the idea, to which all normal biologists are sympathetic, that living cells are directly concerned in lifting water, absorbed by the roots, to the leaves, where it is used or lost. In 1893 Strasburger thought he had refuted this tlheory by amputating leaves, branches, even trees, causing them to absorb water holding copper sulphate, a plant poison, in solution, and observing the ascent of the copper sulphate to the leaves. Overton (1911) believed that he had confirmed Strasburger's conclusion that living cells are not concerned, by killing the cells in a zone around and through leafy stems by means of steam. As indicated by Schwendener (1892), it does not follow from Strasburger's experiments that living cells are not concerned because, throughout his amputated portions of living plants, the cells were alive and presumably active until the poison reached them and, according to Strasburger's own statement as well as common observation, the copper sulphate solution, at first moving rapidly, passed upward more and more slowly as it neared the leaves. This would inevitably be the case if living cells are concerned, for, with the diminishing number of living cells as the poison climbed the tree, the sap stream would be lifted by fewer and fewer cells and hence would move more and more slowly. On the other hand, Overton did not find that steaming his intact Cyperus plants materially retarded the flow of sap as compared with untreated plants, and concluded therefore that living cells could not be concerned. This conclusion would be justified if water goes up a tree onlv as liquid water, either as solid cylinders as the Dixon hypothesis assumes, or as films along the walls of ducts and tracheids. On the other hand, if the water moves to any considerable extent as vapor in otherwise hollow cylinders in the ducts and tracheids, raising the temperature should accelerate the movement, whether living cells are involved or not. Conversely, lowering the temperature should retard the movement. In order to test this inference I made certain experiments out of doors, on intact and otherwise undistuirbed parts of castor bean (Ricinus communis)