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CHAPTEE VI 



THE TEANSPIRATION 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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