ABSORPTION AND MOVEMENT OF WATER 131 



may thereby be reduced, the means of developing turgor 

 will be extended and the total pressure of the organ or of 

 the plant may remain the same. The turgor-pressure may 

 also be reduced by the living cells which abut on the 

 lifeless ducts and tracheids, pressing out water into these 

 otherwise empty shells. Continuing this process of excre- 

 tion into the wood elements may result in pressure develop- 

 ing in them also. This pressure in lifeless cells may justly 

 be called sap-pressure in distinction from turgor-pressure, 

 which is possible only in living cells or in an apparatus 

 similarly constructed. 



Perennial plants in temperate climates exhibit all of these 

 phenomena each year with the return of spring. When there 

 are again sufficient warmth and water, the cells in which 

 starch and other reserve foods were stored for the winter are 

 awakened to new life, and form enzyms needed to convert 

 the starch and other insoluble solids into soluble ones. 

 These go into solution in the cell-sap, which thereby in- 

 creases in density and in osmotic potential. The cell-sap in 

 this condition rapidly absorbs water and, tending to in- 

 crease proportionally in volume, develops a pressure equal 

 to the force needed to keep it at that volume. This pressure 

 will necessarily be exerted upon the adjacent cells, will thus 

 be extended from cell to cell, the substances dissolved and 

 the water dissolving them will pass by osmosis in the same 

 directions, i. e. in the directions of least resistance, mechan- 

 ical or osmotic. Thus the local pressure will be reduced and 

 the danger that the cells may burst will be removed. The 

 total pressure may remain the same, or the local pressure 

 may become so distributed and equalized that in the plant 

 as a whole there will be none. This last is accomplished 

 whenever water is given off, as vapor or liquid, from the 

 surface of the plant in amount equal to that absorbed. 



The ratio between water absorbed and water given off 

 indicates whether there can be any pressure of the cell-sap 

 in any living ceU (turgor-pressure) or in the whole plant 

 (sap-pressure). The greater the absorption in proportion 

 to the loss of water by evaporation, transpiration, or secre- 

 tion, the greater the pressure, local and general ; conversely, 



