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. 



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 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, 



