Conditions and Principles of Absorption 77 



sap above that of the soil solution, or other liquid environ- 

 ment, conditions a turgor, an expression of osmotic force. 

 This turgor is coexistent with growth. It likewise confers 

 upon cells or organs a substantial rigidity. The concen- 

 tration of osmotically active substances manifest through 

 the absorbing surfaces represents a constant pull upon the 

 environment for water, so that root-hairs are able to ab- 

 stract water from surfaces or solutions which do not repre- 

 sent a greater pull. The plasmatic membrane is extremely 

 complex with regard to permeability, and it may exhibit 

 marked powers of selective absorption. In simple (few- 

 celled) plants osmosis and diffusion may be all-sufficient 

 in what is practically the movement of solutions, but in 

 higher plants there are, in addition to these important 

 forces, also other factors affecting mass movement along 

 the special conducting paths of the fibrovascular system, 

 as noted later. 



49. Sap or root pressure. - The absorptive capacity 

 of the root, conditioned by its osmotic relations, may give 

 rise to a pressure, termed root pressure or sap pressure, 

 which may be manifest within the plant whenever the 

 greater rapidity of transpiration does not create a nega- 

 tive tension. 



Bleeding phenomena are evidences of this pressure. 

 During the spring, in particular, the maple, birch, grape, 

 potato, black nightshade, nettle, and a variety of other 

 woody and herbaceous plants bleed profusely. In some 

 cases bleeding is checked by drying-out, by the deposition 

 of solid or glutinous matter, and by growth processes (ty- 

 loses) filling up the vessels from adjacent cells. In other 

 instances corky layers may be formed sooner or later. 



