84 THE OAK 
to the branches, petioles, and leaf-venation—always in 
the wood—and is finally distributed to the mesophyll 
cells, which absorb it and evaporate the greater part of 
the water into the intercellular passages communicating 
with the outer air through the stomata. 
Two points need notice here. The first is that this 
absorption and evaporation in the mesophyll constitute 
a cause of the upward movement of the water in the 
vascular bundles—a movement which is propagated 
through the whole stem until it makes itself effective 
even in the roots. The exact mechanism of the move- 
ment in the stem itself is too complex for discussion 
here; but I may sum up the matter by saying that the 
disappearance of the water at the surfaces of the leaves 
starts a series of flows in directions of least resistance 
towards the mesophyll, and as long as the evaporation 
goes on more water flows into the cells, to replace that 
lost, from the vessels of the stem, when the water- 
columns are supported and moved partly by capillarity 
and by the air bubbles in the cavities, and partly by a 
peculiar co-operation of the living cells of the medullary 
rays. ‘The second point referred to above is that the 
evaporation from the mesophyll cells will be the more 
rapid in proportion as the air outside is drier and the 
stomata wide open; and the more energetic this 
evaporation is, the more salts the mesophyll cells will 
acquire in a given time, because, of course, the salts do 
not pass away in the evaporated water but are left in 
the cells. It has been calculated that an oak-tree may 
