Page:Popular Science Monthly Volume 59.djvu/506

496 matter, and the way is blocked; the canal is filled with débris, so to say, and has become disused. Again, the old central wood frequently decays until there is only the outer ring of the later-formed wood remaining with the bark that covers it. That the bark is not the water-carrier may be shown by removing a ring of it and thus breaking the connection without interrupting the upward flow. That it does pass through the young wood may be shown by cutting this portion without harming materially the bark or the heart wood, when the leaves quickly wither and the tree may die. In short, the sap-wood is well named, as through it the soil-water mounts upward from the roots to the leaves.

In many plants, however, there is no well-developed ring of wood. Either the stem is too young to have one or its construction such that it never appears. However, the same kind of tissue is somewhere to be found in the stem, usually in strands or portions of tough threads, as in the corn-stalk, and through these the crude sap is transported. Some of these succulent stems are so transparent that they admit of experiments which demonstrate both the path and the rate of the upward flow. For example, a balsam stem may be cut and, while fresh, plunged into a harmless colored liquid, as that of some aniline dye. It is found that the woody bundles are the first to take the stain and that it mounts upward with a rate that is an index of the flow of sap and may be some feet in a single hour. Another test for the rate is found in the use of a harmless salt, easily detected in extremely minute quantities by the spectroscope. Let it be lithium nitrate, for example, and its rise discovered by making sections of the stem at different distances and burning small fragments.

But having determined the place of entrance, line of ascent and point of departure of the aqueous stream, it by no means follows that all the forces have been named that bring about the transfer. That living plants carry water and make it one of the chief labors of all their active days is beyond question, but physicists and physiologists, chemists and biologists are as one concerning the mystery that here exists. A grape-vine stump bleeding in early spring is a stumbling block for them all, and they fall back upon 'root pressure,' a term more convenient for covering much ignorance than for service as a full, well-rounded explanation of the phenomena in question. Membrane diffusion will account for much, capillary attraction helps considerably, and the differences of gas pressure within and without the cells, as in the tapped sugar maple in early spring, count for something; but back of all is a vital force that has not been reduced to a physical or chemical basis.