THE TRANSPIRATION CURRENT 79 
indeed, it will be coloured quite up to the veins of the leaves, 
while the pith and cortical tissues will remain unstained. 
An isolated branch can be taken as the subject of the experi- 
ment, its cut surface being placed in a solution of the dye. 
The dye in these cases passes with the current of water, 
as may beseen by the difference in its rate of passage when 
transpiration is vigorous, and when from severance of the 
leaves of a branch it can penetrate only by diffusion. 
A good deal of controversy has been excited with refer- 
ence to the manner in which the transport of the water in 
the wood takes place. Sachs originally suggested that the 
path was altogether the walls of the cells, and that their. 
cavities were empty. This view was based partly on the 
fact that the vessels undoubtedly contain a quantity of air. 
during the period of active vegetation, and that this air is 
at a less pressure than that of the atmosphere. Another 
reason advanced for it was based on the nature of lignin. 
and its relation to water. While refusing to absorb much 
water and swell as cellulose can be made to do, lignin can 
contain a certain quantity, which it will part with very 
easily. On this view the walls of the lignified vessels may be 
regarded as a column of water held together by the mole- 
cules or micelle of lignin. A very little water removed 
from the top of such a column would be immediately 
replaced from below so long as a supply existed there. 
Such a remarkable conductivity, however, is probably 
not possessed by the walls of the vessels. Many observa- 
tions made in recent years tend to negative this view, and 
to support the hypothesis that the water passes in the 
cavities of the vessels. Sachs’s opinion that these are 
always free from water during active transpiration has 
been shown not to be well founded, for various observers 
have proved that their cavities are occupied by a chain of 
water-columns and air-bubbles, the air having been 
originally absorbed from the intercellular space system. 
If the end of a transpiring branch is injected for a short 
distance with a viscid fluid, which will penetrate the 
