THE MECHANISM OF ACTIVE EXUDATION 269 



the cells which surround the intercellular space into which water is excreted 

 are themselves active, or whether water simply filters through them. In the 

 pitcher of Nepenthes the water is excreted into an external cavity. 



All such excretion of water is due to the activity of living cells, and 

 need not necessarily be always produced in the same manner (Sect. 41). 

 Besides this active excretion under pressure, a passive or plasmolytic 

 excretion of water is possible. In the former case the escape of water 

 from the cell is due to the internal character and activity of the protoplast, 

 while in the latter case the water is withdrawn from the cells by the 

 osmotic attraction of external substances 1 , as occurs, for example, in 

 nectaries (Sect. 49). Indeed, provided the cell-walls are permeable to 

 water, the presence of sugar or any other soluble and highly osmotic 

 substance on the outer surface of a cell must necessarily cause water to be 

 withdrawn until a condition of equilibrium is restored. So long as the 

 osmotically active substance remains outside, water will continue to be ex- 

 creted even after the plant becomes flaccid ; moreover, evaporation will 

 tend to concentrate the external solution, and hence to exert a greater 



o 



osmotic attraction upon the diminishing percentage of water still contained 

 in the cells with which the osmotic solution is in contact. 



This is therefore a process which is regulated by the purely physical 

 laws of osmosis and osmotic action. Hence a trifling difference of con- 

 centration in the fluids touching the opposite sides of a cell, or of a plate 

 of tissue composed of living cells, is sufficient to induce a filtration of water 

 even against marked pressure. A difference of concentration equivalent to a 

 oi per cent, solution of potassium nitrate corresponds to the pressure of 

 a column of mercury 27 centimetres high, while a difference of i per cent, 

 will drive water through the cells with a force equivalent to a pressure of 

 3-5 atmospheres (cf. table, p. 146). 



In nectaries the secretion and excretion of sugar is undoubtedly of 

 primary importance, the excretion of water following as a necessary con- 

 sequence ; bleeding, on the other hand, is due to an intracellular secretion 

 under pressure by special active cells. This must necessarily be the case 

 whenever pure water or an extremely dilute solution is excreted, for other- 

 wise no marked bleeding-pressure could be attained. The bleeding-pressure 

 of a vine stem often exceeds that of one atmosphere, although the sap holds 

 hardly any more dissolved substance than river water does 2 . 



A plasmolytic excretion of water can continue only so long as the 

 higher concentration of the extracellular fluid is maintained, and hence 



1 See Pfeffer, Studien z. Energetik, 1892, p. 265. 



* linger (Sitzungsb. d. Wien. Akad., 1857, Bd. xxv, p. 444) found that the specific gravity of 

 the sap bleeding from a vine may fall as low as o-oooi. A saltpetre solution of the same specific 

 gravity would contain about 0-016 per cent, of salt. The detailed composition of the sap obtained 

 by bleeding is given by Ravizza, Bot. Jahresb., 1888, p. 68. 



