1916] BAILEY—ASCENT OF SAP 137 
gelatine pass through the septa of the bordered pits, the walls 
of the tracheids must absorb them. In view of the perforated 
structure of the pit membranes in larch and in other conifers, and 
JONnsson’s observations upon the sievelike membranes in the vessels 
of angiosperms, these criti- 
cisms of the work of ELFVING, 
ERRERA, and STRASBURGER 
do not appear to be entirely 
conclusive. 
The presence of bubbles of 
undissolved gas in the wood 
of transpiring plants has been 
considered to be a serious 
objection to the cohesion 
hypothesis of the ascent of 
sap in plants; for it has been 
admitted by Drxown (l.c.) 
that a bubble, having a 
diameter of 0.02mm. or more, 
would destroy the possibility 
of tension in any tracheid or 
vessel which is located more 
than a few feet from the 
ground. Drxon avoids this 
difficulty by stating that, 
although there is undoubt- 
edly considerable undissolved 
air or gas in the sap wood 
of plants, not all of the 
tracheids or conducting ele- 
ments contain bubbles; that Fic 2.—Longitudinal tangential section 
is, a large number of tra- of sagen? =| — oi gpg nye aa 
cheids may be blocked by air, seats mre of the aera ace flied with 
yet all the continuous water air; after Dix 
columns in the wood need 
not be broken (text fig. 2). This is considered to be due to the fact 
that the membranes of the bordered pits prevent undissolved gas 
from passing from one element into adjoining ones. 
