86 VEGETABLE PHYSIOLOGY 



has been found. It is a little difficult to measure in most 

 cases ; the plan generally adopted has been to 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. McNab 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 definite intervals, 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-opera- 

 tion 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, together with 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, capillarity has 

 been suggested as playing a part. This cannot, however, 

 have much effect in a system of closed trachei'ds, 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 



