THE TRANSPIRATION CURRENT 85 
from the intercellular space system. If the end of a transpir- 
ing branch is injected for a short distance with a viscid 
fluid, which will penetrate the cavities of the vessels and 
subsequently solidify, these passages can be occluded for 
a distance of a few centimetres. Gelatin or paraffin can 
be used for the experiment, being injected at a moderately 
low temperature such as will not injure the vitality of the 
tissue. If after it has solidified a fresh surface is made by 
a clean cut a very short distance from the end, and the 
branch immersed in water, the leaves very soon flag, even 
if some pressure is applied to the water in contact with the 
cut surface. If the path of the liquid were the cell-walls, no 
obstacle being offered to the transfer of water to them, the 
upper portions ought to remain turgid. The experiment 
shows that the normal channels are blocked by the paraffin 
or gelatin used, and flagging results from the obstruction. 
A similar demonstration that the water passes by the 
cavities or lumina of the cells is afforded by the experi- 
ment of compressing the stem in a vice; if the pressure is 
carried so far as partially or entirely to obliterate their 
cavities, the rate of flow is materially interfered with. 
The progress of a dye injected into the surface of a cut 
branch also points to the same conclusion. If such stains 
as fuchsin or eosin, which colour wood very rapidly, are 
forced up into a stem and sections made almost immediately, 
the lignified walls will be found to be in process of staining, 
and the colour will be seen to be deepest on the side of the 
wall abutting on the lumen, often only penetrating partly 
through the thickness. If the wall itself were the path 
of the pigment solution, its thickness would be stained 
uniformly as far as the dye penetrated at all. 
The rate at which the transpiration current naturally 
flows varies a good deal, plants showing differences among 
theinselves as to facilities of transport. In a fairly vigorous 
tree it may be taken to be about 1-2 metres per hour, though 
in some plants it has been observed to be three times as 
rapid. In other cases as low a speed as 0-2 metre per hour 
