270 THE MOVEMENTS OF WATER 



the conditions necessary for such an excretion can only be maintained 

 with difficulty in the neighbourhood of cells which actively excrete water. 

 The transitory effect of the application of a concentrated solution to a plant 

 is shown by the fact that an active exudation of water is only temporarily 

 diminished or stopped by placing the root system in a solution of potassium 

 nitrate (Sect. 44). Bearing in mind this behaviour, it is easy to see why 

 Wieler l was unable to induce any bleeding from inactive decapitated plants 

 by filling the tube attached to the stump of the stem with solutions of salt- 

 petre of from oi to i per cent, concentration. 



On these grounds we may with comparative certainty conclude that even 

 when the escaping sap is rich in dissolved materials (Sect. 43), the exudation- 

 pressure is still produced by the excretion of water under pressure from 

 the active cells, and that the sugar present in the sap is a food material in 

 process of translocation, and is not formed in order to excite any plasmolytic 

 excretion of water. Dissolved substances will naturally influence the 

 exudation of water according to their distribution and osmotic energy, and 

 it is possible that in certain cases both an active and a plasmolytic excretion 

 of water may co-operate in producing the exudation-pressure. 



The means by which intraccllular activity is able to cause an active 

 excretion of water have not yet been discovered, and since the same end 

 may be attained in various ways, it is impossible to make any theoretical 

 deductions as to what the nature of the process must be ''. The escape of 

 water from the active cells may be produced in a variety of ways, and may 

 also be possible in one direction only. Thus a particular distribution of 

 the osmotically active cell-contents may be maintained, or changes in the 

 pressure exerted upon the enclosed cell-sap may occur, due to pulsations 

 or active contractions. Alterations in the internal pressure are of common 

 occurrence, and these might exercise a pumping action in a particular 

 direction under certain special conditions ; but it does not follow that the 

 bleeding-pressure is necessarily produced in this manner, nor is it of necessity 

 coupled with the existence of pulsating vacuoles. Moreover, no exudation- 

 pressure would be produced even by these means if backward filtration 

 were possible, as is the case when water is excreted into the intercellular 

 spaces of a stimulated staminal filament of one of the Cynareae, or into 

 those of a stimulated pulvinus of Mimosa. 



When an active exudation of water is artificially induced, the excreting 

 cells have to be aroused by the action of an external stimulus. This stimu- 

 lating action is shown very markedly by the glands on the leaf of Dionaca 

 innscipula, which are caused to excrete an abundance of water (along with 

 pepsin and a little acid) by certain chemical stimuli, and which return 



Wieler, 1. c., p. 163. 



Cf. Pfeffer, Energetik, 1 892, p. 265, and Osmot. Unters., 1877, p. 223 ; also \Vieler, 1. c., p. 151. 



