78. VEGETABLE PHYSIOLOGY 
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 
transpiration 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 continuity of 
the cortex and that of the pith are allowed to remain 
intact, the leaves very speedily droop and become flaccid. 
Ifa 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 in 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 
colouring matter will be found to have stained the wood 
for a considerable distance; in the case of a small plant, 
