66 VEGETABLE PHYSIOLOGY 



In such tissues as those just mentioned we can demon* 

 strate with ease what is more difficult to detect in the 

 others, that not only is water admitted to the cells, but it 

 is also given off from them. This does not depend on 

 osmosis in the stem or leaf, but is due to evaporation, 

 which takes place from the surfaces of the cells abutting 

 on the intercellular spaces, whence the watery vapour is 

 exhaled through the stomata, or, in the case of a woody 

 stem, through the lenticels. In a cell surrounded by water 

 such removal must depend upon osmotic currents. 



This removal of w.ater occasions a need for a continuous 

 replenishment of the liquid in the vacuoles, which is 

 brought about by the same modified osmosis which has 

 been described. We can see that this process must be 

 continually taking place in a complex of succulent cells. 

 If we consider two which are contiguous and are separated 

 from each other by a common cell-wall, it is evident that 

 unless the proportion of water to dissolved substances in the 

 vacuoles of both is the same, a flow of water from one 

 to the other will take place till this equilibrium is reached. 

 Any disturbance taking place in one cell of a complex will 

 hence spread from cell to cell until the composition of the 

 fluid contents of them all is uniform. When we consider 

 the differences, sometimes very slight, sometimes more 

 extensive, which are continually taking place in the meta- 

 bolic activities of the separate cells of a community, it is 

 evident that, so long as life lasts, currents of this kind 

 must be continually passing from cell to cell in various 

 directions, and frequently at very different rates. 



Evaporation from a cell into an intercellular space 

 must lead to a certain increase of the concentration of the 

 solution of osmotically active substance in its vacuole. 

 This then attracts water from the contiguous cells, and 

 consequently, independently of metabolic changes affecting 

 the quantities of such osmotic substances, evaporation itself 

 must help in causing movements of water from cell to cell. 



The quantity of these osmotic substances which are 



