Page:The Oak.djvu/102

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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 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 movement 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