566 PHYSIOLOGY. 



Sect. 5. DIFFUSION OF FLUIDS IK PLANTS. 



Diffusion in Aquatic and Cellular Plants. In aquatic plants 

 the entire surface is employed in absorption ; and the liberation 

 of gases in the respiratory or other processes being accompanied 

 by condensation of the cell-contents, endosmotic action is kept up 

 constantly during active vegetation. 



In the Cellular plants, such as Lichens, Fungi, and even in 

 Mosses and Hepaticse, the diffusion of the fluids would appear to 

 be a result of simple endosmotic action continued from cell to cell 

 in more or less complex series ; and in plants growing in air, 

 evaporation of gases increases the density of the contents of the 

 last or uppermost cells of the chain. 



Diffusion in Vascular Plants. In plants with well-developed 

 stems and roots, the liquid nutriment is absorbed by the latter, 

 and the movements which the absorbed fluids have to make are 

 much more complex, not only from the greater variety of forms of 

 tissue through which they have to pass, but from the multiplied 

 details of the interchanges with elaborated matters arising from 

 the scattered distribution of the leaves over the axis. 



Ascent of the Sap. As so large a quantity of water is absorbed 

 by the roots from below, it is clear that the diffusion of that fluid 

 (or sap, as it is now called) must in the first instance be in an 

 upward direction ; hence the phrase ascent of the sap. The main 

 current of the watery sap, is upwards from the root, through the 

 stem and branches, to the leaves, wherein, owing to the changes 

 it there undergoes and which will be hereafter alluded to, its cha- 

 racter becomes altered and the direction of its current is varied 

 according to the requirements of different parts of the plant, &c. 



The term sap is retained for convenience' sake ; but it must be remem- 

 bered that there is no homogeneous fluid, either ascending or descending, 

 crude or elaborated, of similar constitution in all parts of the plant, cor- 

 responding to the blood of animals. The sap varies in constitution in 

 different parts of the same plant at the same time. In like manner there 

 is no continuous system of tubes in which sap could "circulate." In 

 sprino-, when vegetation is most active, or, at other times, when special 

 circumstances favour growth in particular places, a current of watery sap, 

 containing relatively little of the matters formed in consequence of leaf- 

 action, is specially manifest ; and as the ends of the shoots and buds are at 

 this period centres of activity, so the flow is mainly an upward one. In 

 autumn, when consolidation of tissues and storage of nutritive matters are 

 the chief operations of the plant, there is an increased necessity for the 

 presence of matters formed in consequence of leaf-action, and the flow is 

 to a large extent a downward one. But there is no absolute difference 

 between crude and elaborated saps, and no absolutely fixed course for 

 them to take. The ascending sap, so called, which is so manifest in 



