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. Recent 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 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 
