Absorption. 559 



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 pas- 

 sage 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 par- 

 ticularly 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 



