This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
- This term is employed to designate the act by which a plant draws the materials necessary for its growth and sustenance from the soil and air. All parts of a plant contribute to the fulfilment of this function, or at least so long as they are young and herbaceous. But the root is the principal channel for the conveyance of the various constituents which go to build up a plant. And the tender extremities (spongioles) of their fibrils or ramifications are the most active points. Leaves, too, are provided with numerous minute openings, termed stomates, which, according to the state of the weather and the amount of moisture contained in the plant, are either open or closed.
The elements taken up by plants through these two channels are either in the gaseous or liquid state, for not the minutest particles one could imagine to be held in suspension by water can enter. It may readily be conceived that very fine, almost impalpable grains of dust may mechanically penetrate the stomates, but it does not follow that they are absorbed. On the contrary, they obstruct and destroy these passages and prevent the leaves from exercising their physiological functions in a regular manner, and consequently the health of the plant becomes impaired. This effect is well-known to gardeners, especially on window and conservatory plants, and on those in the open air near public roads, which they obviate by frequently syringing, or otherwise the plants would inevitably be choked. In the natural order of things the rains are sufficient to accomplish this purpose. The action of the roots is exercised especially on the water contained in the soil, principally in its ordinary liquid state. This water is never quite pure. By virtue of its dissolving qualities it is more or less charged with various foreign matters, the most important of which for vegetation are the salts of potash and soda, the phosphates and carbonates of lime, and ammoniacal and carbonic acid gases. Brought into contact with the constantly renewed cellules of the spongioles, they enter and are transmitted through all the ramifications of the plant. These spongioles act as perfect filters, permitting the passage of materials held in solution, but barring it effectually to the corpuscles that are merely held in suspension by the fluids. The circulation of these fluids from cell to cell through the plant is effected by a process termed endosmosis, and dependent upon a difference in the density or chemical composition of the contents of the neighbouring cells, which causes a current to set in through the permeable partitions of the cells, and continue so long as there is a disparity in their contents. The amount of evaporation from the leaves governs to a certain extent the flow of the sap. Neither the cells of the spongioles nor of any of the tissues which the absorbed water traverses are empty, for they already.contain liquids charged with diverse substances, principally sugary matters. The water pumped up from the soil mixes with these liquids, and becomes thereby what is termed the crude or ascending sap, in contradistinction to the elaborated or descending sap. It receives the latter designation after it has been assimilated, or undergone important alterations by exposure to atmospheric influences in the leaves, and rendered fit for the alimentation of the plant. It is scarcely necessary to explain that the ascending course of the sap is not exactly the same in all vegetables, but varies according to the structure of the species. In Dicotyledons or Exogenous plants, and particularly in trees, where it has been more carefully studied, it rises through the young wood or alburnum; and the assimilated sap descends through the inner layers of the bark. Sap rises, everything else being equal, in proportion to the number and size of the conducting channels. This effect is more easily seen in plants with slender scandent stems, like the Vine and Ivy, which can climb to the summits of lofty trees or buildings. In the majority of these plants there is a great development of foliage, and consequently a large quantity of moisture is lost by transpiration, especially when the atmosphere is very dry. To keep up a corresponding supply there must be free and rapid circulation, which is the case in the Vine, as everybody knows who has cut an active growing cane, and observed the copious discharge of sap. A transverse section of the Vine will show these vessels even without the aid of a lens.
The chief substance useful to man furnished by the sap of plants is sugar, which is produced by such diverse plants as the Sugar-cane, Beet, Sugar Maple, and many Palms, etc.
As we have previously observed, the leaves and other green parts of plants are also organs of absorption; but their action is limited to the imbibition of aqueous vapour and the gases contained in the air, principally oxygen, carbonic acid, and ammoniacal gases.