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 water 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 
