8 TRANSPIRATION AND ASCENT OF SAP ch. 



extremely rapid action seems only explicable by assuming 

 some sudden change in the permeability of the proto- 

 plasm. 



Secretion of water. The active secretion of water 

 by cells is particularly well illustrated by the water glands 

 in the tips of the leaves of Colocasia antiquorum. As is 

 well known, when this aroid is under suitable conditions 

 of moist soil, saturated atmosphere, and favourable 

 temperature, a succession of drops (often as many as two 

 per second) may be seen issuing from its leaf tip. A 

 similar exudation of fluid may be often observed on the 

 leaves of grass-seedlings when surrounded with a satu- 

 rated atmosphere. 



It is evident that this exudation of fluid would be easily 

 explicable on purely physical processes if the fluid con- 

 tained any considerable quantity of dissolved substances. 

 Then the external solutes might be assumed by virtue of 

 their osmotic pressure to extract water from within and so 

 keep up the supplies on the outside of the cell. The pres- 

 ence of dissolved substances may be sometimes demon- 

 strated in the exudations of the fungi by evaporating 

 drops of the fluid on polished glass. How far these dis- 

 solved substances are crystalloids, and so competent to act 

 osmotically, remains to be determined. At the same time, 

 it should be noted that, even admitting there are sufficient 

 crystalloidal solutes on the surface of the cell to account 

 for the exudation, it seems we must assume a secretory 

 action in the protoplasm to bring these crystalloids out 

 of the cells and expose them on the outer surface, and this 

 action must be a continued one as the exudation will 

 constantly be carrying off its solutes. 



If, however, the fluid exuded is pure water, or contains 

 practically no dissolved crystalloids, it is evident that 

 osmosis cannot account for the process, for the osmotic 

 action of the solutes within the vacuoles cannot cause the 

 elimination of water on the outside of the cells. In this, 



