THE TEANSPIBATION CUEEENT 81 



immerse the cut ends of branches in a solution of such a 

 dye as eosin, and notice how far the dye penetrates in 

 some unit of time. The objection to this method is that 

 very frequently the water of such a coloured solution will 

 travel faster than the dye dissolved in it. Sachs used 

 instead a solution of a salt of lithium, which he found was 

 free from this objection. He detected the rate of progress 

 of the lithium by means of spectroscopical examination* 

 ascertaining how far the metal could be traced in the stem 

 when pieces were cut out and burnt after a definite time, 

 during which absorption had proceeded. 



The causes of the transpiration current are not fully 

 known, but there is no doubt that it is due to the co- 

 operation of many factors, not one of which by itself is 

 sufficient to account for it. Two of the main influences 

 which are at work have been incidentally alluded to, which 

 must now be discussed in greater detail. These are the 

 constant pumping action of the cortex of the root, giving 

 us the force known as root-pressure, and the evaporation 

 into the intercellular spaces, and its exhalation from the 

 surfaces of the green parts of the plant, which we have 

 spoken of as transpiration. Eecent investigations make it 

 probable that we must add to these the force of osmosis in 

 the parenchyma of the leaves, which apparently brings 

 about the passage of the water from the veins into the cells 

 of the leaf-substance. 



Besides these, other factors have been held to co- 

 operate, though much less certainly than they. The walls 

 of the vessels having an extremely narrow calibre, capil- 

 larity has been suggested as playing a part. This cannot, 

 however, have much effect in a system of closed tracheids, 

 like those of the secondary wood of the Conifers, which, 

 nevertheless, conduct the water. It has been thought 

 that the living cells of the parenchyma, which abut upon 

 the woody tissue of the stele, may play a part similar to 

 the pumping action of the root. The medullary rays of 

 the stele in tall tree trunks have been held to play a 



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