THE TRANSPIRATION CURRENT 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. 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, 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 
6 
