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, an] consequently a.large quantity of moisture is lost by 
