ABSORPTION AND TRANSMISSION OF WATER 107 



(4) Summary of water loss. By evaporation pure water 

 is lost from the plant. Thus the osmotic pressure of the 

 fluids in the leaves and near evaporating surfaces is increased 

 and other water diffuses to these regions, thus tending to 

 re-establish an equilibrium of diffusion tension. Water is 

 eventually taken from the xylem vessels and, since these are 

 not lined with protoplasm, a mass movement of water is set 

 up, flowing upward through the xylem strands. This carries 

 with it whatever dissolved substances have been extruded 

 into these strands from the roots. Evaporation cools the 

 leaves. Leaves, etc., are able to extrude solution upon their 

 surface through specialized openings, the glands and water 

 pores. The exact process by which this extrusion takes place 

 is not known ; it is probably an osmotic one, perhaps coupled 

 with some periodic change in permeability of the protoplasts. 

 Leaf cells will act in the same way in the opposite direction 

 without regard to water pores (Pitra, Molisch). 



Root cells are able to take in water and solutes and then 

 to pass them on into the xylem. It seems that sap pressure, 

 whether in roots or in wound tissue, must be explained along 

 the same general lines as the action of glands and water pores. 

 Whether a periodic external exudation occurs in roots is not 

 known, but it seems not improbable that at some times roots 

 may let out solutes. Indeed, Czapek 1 and Molisch 2 have 

 observed extrusions from root hairs which are not unlike 

 those from water pores. It may be that some sort of a peri- 

 odic extrusion of solutes is a fundamental property of proto- 

 plasm. If this be true, it is a phenomenon not unlike that 

 exhibited in pulsating vacuoles. 



b) The upward movement of water in trees and other 

 tall plants. Owing to almost insurmountable difficulties in 



IF. CZAPEK, "Zur Lehre von den Wurzelausscheidungen," Jahrb.f.wiss. Bot., 

 Vol. XXIX (1896), pp. 321-90. 



2 H. MOLISCH, " Ueber Wurzelausscheidungen und deren Einwirkung auf orga- 

 nische Substauzen," Sitzungsber. d. kais. Akad. d. Wiss. zu Wien, math.-nat. hist. 

 Klasse, Vol. XCVI (1887), pp. 84-109. 



