TRANSPIRATION-CURRENT 199 



from the fact that many trees carry on their functions after the 

 pith is destroyed and the centre has become hollow and de- 

 cayed. 



It can also be readily shown that the bark and bast do not 

 conduct the rapid upward current, for after a narrow ring-like 

 portion of tissues, as far as the cambium have been removed all 

 round a branch, the leaves above the place where the bark and 

 bast have been cut away do not wither. 



By various experiments it has been proved that the current 

 travels in the youngest or outermost annual rings of woody stems 

 and apparently in the greatest amount, if not entirely, in the 

 cavities of the vessels and tracheids ; the heart-wood does not 

 conduct water but acts as a mechanical support. 



By placing the cut stems of herbaceous plants and the petioles 

 of leaves in coloured solutions of certain dyes, and subsequently 

 making sections of the stems at intervals, and by holding the 

 leaves up to the light, it will be observed that the solutions 

 have travelled along the vascular bundles which have become 

 stained, the rest of the tissues remaining colourless for a long 

 time after the bundles have been coloured. 



The cause of the movement of the water through plants, or the 

 force which propels the transpiration-current, has been the subject 

 of very extensive research for more than a century. 



No adequate explanation can, however, be given which will 

 meet all the facts of the case. The osmotic action of the living 

 cells of the root and stem which results in ' bleeding-pressure,' and 

 the osmotic attraction of substances within the parenchymatous 

 cells of the leaves, which results in a sucking-force withdrawing 

 water from the vascular bundles, help to set up rapid movement 

 of water in a plant. 



In plants of low stature, these forces depending on the activity 

 of living cells, may be sufficient to account for the movement of 

 the transpiration-current, but the conduction of water to the top 

 of very high trees, cannot be satisfactorily explained at present. 



