84 THE OAK. 



the stem ; here it passes up the xylem to the branches, 

 petioles, and leaf-venation always in the wood and 

 is finally distributed to the mesophyll cells, which ab- 

 sorb 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 evap- 

 oration is, the more salts the mesophyll cells will ac- 

 quire in a given time, because, of course, the salts do 

 not pass away in the evaporated water but are left in 



