404 Dixon and Joly . — The Path of 
and after this has been drawn up by transpiration, pieces of 
the conducting tissues are treated with a second solution 
which forms a precipitate with the first. Examination is then 
made to ascertain whether the precipitate is confined to the 
lumen or to the wall. The recorded results of these experi- 
ments have been that the precipitate is confined to the lumen, 
and it is concluded that therefore the first solution has moved 
in the lumen only. It appeared to us that these experiments 
involve a full investigation of the question as to whether 
in any case it is possible for the precipitate to form in appre- 
ciable quantities in the wall ; for if it cannot, they tell us 
nothing as to what went on in the wall. The following 
experiments show the validity of this objection. Thin sections 
(longitudinal and transverse) of Taxus baccata , first soaked for 
some hours in a strong solution of potassium ferrocyanide or 
of ferric chloride, were either dried on the surfaces with filter- 
paper or quickly washed in water, and then treated respec- 
tively with ferric chloride or potassium ferrocyanide. On 
subsequent microscopic examination in no case could any 
more than a faint blue coloration be observed in the woody 
walls (perhaps in part only apparent and due to a thin film of 
precipitate on the cut surfaces of the section), while the lumen 
was choked with the precipitate. On the other hand, the 
cellulose walls of the bast, cortex, and medullary rays were 
deeply coloured, as well as the torus of the closing membrane 
of the bordered pits. In order to obtain denser precipitates 
in the lumina, the sections may be transferred several times 
from one of the mutually reacting solutions to the other. 
That the walls of the tracheides were thoroughly imbibed 
with the solutions there can be no doubt, seeing that the 
sections were very thin. Where the walls were tinted at all, 
the faint coloration was almost completely limited to the 
tertiary thickening layers, just as Strasburger has already 
observed in a branch of Taxus baccata , which stood for some 
days in a solution of potassium ferrocyanide, and was after- 
wards treated in pieces with iron sulphate 1 . 
1 Leitungsbahnen, p. 628. 
