- 1916] BAILEY—ASCENT OF SAP 135 
The experimental evidence, which has been considered to 
demonstrate that the pit membranes are unperforated, consists of 
injection experiments, such as the well known tests of SAcHS with 
mercury and powdered cinnabar. That absence of penetration in 
these experiments may have been due to other factors than the 
unperforated character of the pit membranes does not appear to 
have suggested itself to previous investigators. However, there 
are two possibilities that should be emphasized in this connection. 
In the first place, it is hardly to be expected, a priori, that mercury, 
alloys of low melting points, and similar substances should pen- 
etrate the pit membranes, even if they were porous or sievelike in 
structure. Owing to the small size of the tracheids and bordered 
pits in conifers, and the valvelike action of the tori, the very high 
surface tensions of heavy liquids would tend to prevent their pene- 
tration. In the second place, the structure of coniferous wood, 
particularly that of Taxus, is such that injection masses contain- 
ing finely divided particles (for example, gamboge, India ink, 
cinnabar, etc.) would be very likely to clog the pits or to deflect the 
tori before they could succeed in passing through the minute 
openings in the pit membranes. This would be true even if the 
particles were considerably smaller than the perforations in the 
membranes, and is well illustrated by the frequent failure of India 
ink to penetrate the narrow cavities of ‘‘summer” tracheids when 
portions of the walls of the latter have been cut away. 
In selecting-an injection mass, therefore, it is very essential 
that its surface tension and viscosity should be similar to that of the 
sap in trees, and, if it contains a finely divided solid, the particles of 
the latter should be of such a size and in such numbers that clogging 
and deflection of the tori are reduced to a minimum. 
That the failure of various investigators to secure penetration 
with suspensions may have been due to clogging rather than to the 
unperforated character of the pit membranes was suggested by 
the following experiments. Pieces of the wood of various conifers 
were immersed in India ink and subjected alternately to pressures 
of +10 and —10 Ibs. In this way the tori were continually 
deflected from side to side to facilitate penetration. Figs. 1 and 4 
show the dark colored injection mass passing from one tracheid 
into an adjacent one through the bordered pits. 
