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CHAPTER VI 
THE TRANSPIRATION CURRENT. ROOT-PRESSURE. 
TRANSPIRATION 
In terrestrial plants, so long as circumstances are favour- 
able to the vital activity of the organism, we have, as we 
have seen, a stream of water passing from the roots through 
the axis to the green twigs and leaves, where the greater 
part of it is evaporated. The stream, which we have 
spoken of as the ascending sap, is often called the transpira- 
tion current. Its path through the axis of the plant has 
been determined to be the xylem vessels, which are in 
complete continuity from the young rootlets to the veins 
of the leaves. 
In thick tree-trunks, in which the wood can be seen to 
consist of alburnum and duramen, the stream is confined 
to the former. Proof of this can be obtained in various 
ways. If an incision is made all round the trunk of a tree 
and a ring of tissue removed, everything being cut away 
down to the outermost ring of wood, the leaves of the parts 
above the wound continue to be turgid. If, on the other 
hand, the woody cylinder is cut through, while the con- 
tinuity of the cortex and that of the pith are allowed to 
remain intact, the leaves very speedily droop and become 
flaccid. 
If a plant in a pot is watered with a solution of a dye 
which has no noxious action on the protoplasts, the colour- 
ing matter is absorbed with the liquid which the roots take 
up, and its progress can be traced by a subsequent micro- 
scopic examination of the various tissues of the axis. The 
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