84 VEGETABLE PHYSIOLOGY 
colouring matter will be found to have stained the wood 
for a considerable distance; in the case of a small plant, 
indeed, it will be coloured quite up to the veins of the 
leaves, while the pith and cortical tissues will remain un- 
stained. An isolated branch can be taken.as the subject 
of the experiment, its cut surface being placed in a solution 
of the dye. 
The dye in these cases passes with the current of water, 
as may be seen 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 lower pressure than that of the atmosphere. Another 
reason advanced for it was founded on the nature of lignin 
and its relation to water. While refusing to imbibe 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 molecules 
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 observations 
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 
