the Transpiration-Current. 405 
In order to compare the behaviour of another colloid with 
that of the woody cell-wall, some gelatine was dissolved in 
a strong solution of potassium ferrocyanide, so as to form when 
cool a very stiff jelly. A drop, of this when melted was placed 
on a microscopic slip and covered with a circular cover-glass. 
When the gelatine had set the slip was transferred into a solu- 
tion of ferric chloride, which slowly diffused through the 
gelatine. On microscopic examination no precipitate, if we 
except a faint coloration, could be detected in the gelatine. 
The precipitate was limited to the surface of the gelatine 
at the edge of the cover-glass and to the dendritic cracks 
formed within the gelatine by contraction under the cover- 
glass. In the substance of the gelatine itself x \ Leitz failed to 
show precipitated particles. 
As a modification of this experiment thin pieces of solid 
gelatine were steeped for twenty-four hours in one or other of 
the salt-solutions, and after their surfaces were dried with 
filter-paper, so as to avoid carrying any of the first solution 
which was not contained in the gelatine, they were transferred 
into the reacting solution. While in the latter a film of dense 
blue precipitate formed over the surface, and sections of the 
gelatine showed that ultimately after long steeping in the 
second solution a pale blue coloration penetrated the colloid. 
In view of these results we think the experiments on plants 
cannot be relied on to prove that the salts did not in part 
move in the walls, although they certainly support the idea of 
a ready and free passage in the lumen. 
Choking the Lumen by the Introduction of Foreign Substances. 
Experiments in which flagging of the branch occurs upon 
artificial stoppage of the lumen have been relied on as 
strong evidence in favour of the lumen as the water-way in 
the tracheal tissue. It was objected to Elfving’s experi- 
ments that cocoa-butter was of too greasy a nature and might 
enter the wall, and gelatine was substituted by Errera and 
Strasburger. We confess that we would have had less appre- 
