2 68 



THE MOVEMENTS OF WATER 



r 



SECTION 46. The Mechanism of Active Exudation. 



An active exudation of water is only possible when living cells force 

 water with sufficient energy into the tracheal elements or into the inter- 

 cellular spaces, and the character of the processes involved may be easily 

 understood by reference to the structure and properties of such living cells. 

 Thus, in the accompanying schematic diagram (Fig. 36), if water is forced 

 in sufficient amount into the air space (//) by the surrounding cells, fluid will 

 escape when the air space is opened, while if a manometer is placed in 

 connexion with it, the mercury will be driven upwards until the backward 

 filtration caused by the increasing pressure just counterbalances the ex- 

 cretory activity of the living cells, so that a condition of equilibrium is 

 reached. The pressure will necessarily be less marked if 

 a number of the cells bordering upon the air-space are 

 inactive, for less water is pumped in a given time into the 

 latter, and a backward filtration through the inactive cells 

 may readily occur. When water can undergo backward 

 filtration through certain cells more readily than others, an 

 internal circulation of water may be produced (Sect. 43). 

 The attainment of a high bleeding-pressure is favoured when 

 the inactive cells (the zone c in Fig. 36) are comparatively 

 impermeable to water. As a general rule, however, the re- 

 sulting bleeding-pressure will hardly attain a height corre- 

 sponding to the intensity with which the most active cells 

 excrete water. 



To produce an exudation-pressure it is not necessary 

 that the active cells must border upon the tracheae or the 

 spaces in which the water accumulates, although such a position is ob- 

 viously advantageous. The living cells of the wood, medullary rays, and 

 wood parenchyma are actually able to excrete water, for both stems and 

 roots may exude it after their cortex has been removed '. It is, however, 

 not yet certain whether the epidermal cells of the root which primarily 

 absorb water are themselves able to exert an exudation-pressure, or indeed 

 whether they aid at all in producing it. 



The tracheal elements, as the normal conducting channels, are easily 

 able to transfer the water forced into them to points far removed where 

 exit is possible. The exudation-pressure of water-glands is produced in a 

 manner essentially similar to that of ordinary bleeding, no matter whether 



FIG. 36. 



1 See the experiments on the bleeding of barked twigs, Sect. 42. That the roots of Richardia 

 aethiopica continue to bleed after the cortex has been removed is certain. Wieler, Cohn's Beitrage, 

 1893, Bd. vi, p. 46. 



