ABSORPTION AND MOVEMENT OF WATER 127 



increase with the absorption of water. Such increase in 

 volume is only feebly resisted by the mechanically weak 

 protoplasm. It can be resisted only by the cell-wall, a 

 strong, elastic, permeable membrane, composed of one of the 

 celluloses. Turgor and sap-pressure result from the resist- 

 ance by the cell-wall to increase in volume. If the sap- 

 pressure in a cell becomes greater than the retaining power 

 of the wall, something will change. The absorption of water 

 may be stopped by modifying the composition of the cell- 

 sap, by exosmosis of the osmotically active salts to other 

 cells, or by chemical change of the salts in the sap ; or water 

 may pass out through the wall in a direction of less pres- 

 sure, either into adjacent cells or out upon the surface of the 

 organ. If none of these things occur and absorption con- 

 tinues, the cell-wall will break. 



The cortical parenchyma cells in the root, in the region 

 where absorption through the hairs is taking place, are 

 under sap-pressure whence the misleading name of root- 

 pressure and consequently force water into the conducting 

 elements, tracheids and ducts, with which they are in con- 

 tact. The same process underlies the action of water-pores. 



Certain weather conditions favor the excretion of water by 

 making it possible to develop the necessary sap-pressure. 

 When the air is warm and moist above a warm damp soil, 

 there will be copious absorption through the roots and pro- 

 portionally little loss of water from the upper parts of the 

 plant by evaporation. Sap-pressure necessarily develops, 

 and if these conditions continue, the pressure will presently 

 exceed the retaining power of some or many cells, water 

 will either filter through or break through the cell-walls. 

 If it break through, a wound is formed, and the escape of 

 liquid from it is bleeding (see pp. 130-36). Such wounds 

 are not altogether uncommon.* 



The filtering of water under pressure through cell-walls 

 does not take place indiscriminately, for the permeability of 

 the walls of the cells composing the different tissues is not 



* See Pfeffer, Pflanzenphysiologie, Bd. I., pp. 255 et seq.; Engl. transl., 

 I., pp. 272 et seq. Strasburger, E. Bau und Verrichtung der Leitungs- 

 bahnen, 1891. 



