138 ; BOTANICAL GAZETTE [AUGUST 
It is evident, therefore, that in a discussion of the tension 
hypothesis, the porous or sievelike structure of the pit membranes of 
conifers deserves careful consideration. 
It has been estimated by Drxon (i.c.) that ‘‘the tension applied — 
to the upper end of water columns, which will be able to raise the 
transpiration stream in a tree, must equal the pressure produced 
by a head of water twice the height of the tree.” In trees 75-300 
feet in height, therefore, there must be forces of 5-20 atmospheres, 
tending to draw air and other gases, through the perforations in the 
pit membranes, into the water-conducting passageways. ‘This 
raises the question: Are the perforations in the membranes of such 
small size that the surface tension of the sap in them will be great 
enough to resist the penetration of gas under forces of 5-20 atmos- 
pheres ? 
Perforations of different shapes and sizes occur in each pit 
membrane (text fig. 1 and fig. 6). The larger openings generally 
are more or less elongated in outline, but many of the smaller ones 
are oval or circular. In the stem wood of Larix laricina the aver- 
age diameter of the openings was found to vary somewhat as 
follows: largest perforations, 3.0; medium perforations, 1.64; 
small perforations, 0.5 yu 
In calculating the knoe tension of liquids i in circular openings, 
the expression of CANTOR® is commonly used. This is as follows: 
rH d?y? 
aes See bs ans 
in which y=surface tension in dynes, r=radius of hole in centi- 
meters, H=air pressure in dynes per square centimeter, and d= 
density of liquid. Assuming a temperature of 15° C., and that 
the pressure of one atmosphere equals 1,013,000 dynes, the pressure 
needed to overcome the surface tension of water in circular per- 
forations of the foregoing diameters should be as follows: small 
perforations, 5.8 atmospheres; medium perforations, 1.8 atmos- 
pheres; large perforations, 1.0 atmosphere. 
That the actual pressures required to overcome the surface ten- 
sion of sap in the perforations of the pit membranes would differ 
6 Wied. Ann. 47:399. 1892. 
