272 THE MOVEMENTS, OF WATER 



Neither a low nor a high osmotic pressure can drive water through a semi-permeable 

 membrane in one direction only. Hence a twofold physical error is involved 

 in the attempt to explain the forcible transmission of water in a particular direction 

 as being simply due to its escape in the direction where least resistance is offered to 

 filtration, as the turgidity of the cell increases 1 . As soon as the full hydrostatic 

 pressure is reached, there is perfect equilibrium on all sides with regard to the entry 

 and exit of water, while before that time, at every point where an exchange was 

 possible, the inward current of water must have been greater than the outward. 



Nor can any difference in character between the two opposite cellulose walls of 

 a cell cause water to be actively forced in a particular direction, although as soon 

 as any difference of potential has been created, the permeability of the cell-wall 

 becomes of importance in determining the direction and character of the ensuing 

 movement of water. Indeed, appropriate differences existing between the enclosing 

 cellulose envelopes of the cells may easily cause the current to take place in 

 a definite direction, and thus lead to an excretion of water. The cell-wall, however, 

 plays even then a purely passive part, and is only indirectly of use as an aid to the 

 active exudation of water. From these considerations it follows that the conditions 

 which create and maintain bleeding are not due to the existence of pits, pores, or 

 permeable areas in the walls either of the active cells or of the tracheae into which 

 the water is forced. The fact that an exudation of water is not always possible 

 shows that a special anatomical arrangement and structure is necessary to pro- 

 duce it, but these can hardly be the actual exciting causes. Moreover, as has 

 already been mentioned, the water may be carried equally well in the opposite 

 direction by the conducting elements. The erroneous suppositions of certain 

 authors that changes of temperature or of the tissue tensions may be the primary 

 agencies in inducing an active exudation of water have already been discussed 

 (Sect. 44). 



SECTION 47. The Excretion of Water from Uninjured Plants. 



Many uninjured plants and plant organs are able to excrete water in a 

 fluid condition, and just as in the case of its exudation from cut surfaces, the 

 phenomenon is due to the activity of living cells, by which water is driven out 

 at certain points from the intact plant. We are brought directly into contact 

 with this excretory activity when the water-excreting cells are on the 

 surface of the plant, as is the case in nectaries, as well as in certain hairs 

 and fungal hyphae. When, however, the cells which excrete water are 

 internal in position, the only evidence of their functional activity is afforded 

 by the water which escapes by means of more or less specialized channels, or 

 through groups of passive cells. This latter case, therefore, closely corre- 

 sponds with the phenomenon of bleeding as regards the manner in which 

 the water escapes ; in part, at least, the excretion of water through water- 



1 See Strasburger. Bau u. Verricht. d. I.eitungsbahnen, 1891, p. 



