1916] BAILEY—ASCENT OF SAP 141i 
9 
of gas, under forces of considerable magnitude, air pressures of 
50-200 lbs. were applied to various conifers. The air passed 
through the membranes in all cases, whether the pressures were 
gradually or suddenly applied. . 
Summary and conclusions 
Exception is taken to statements that the membranes in the 
bordered pits of conifers form complete septa that are entirely 
impervious to finely divided solids and undissolved gases. 
Perforated pit membranes are clearly visible in properly stained 
sections of Larix and Sequoia. Owing to the minute size and tenu- 
ity of pit membranes, their detailed structure is more or less 
obscured by the thick, overhanging secondary walls. 
Aqueous solutions, containing finely divided particles of carbon, 
can be made to pass through the membranes in the bordered pits 
of sap wood taken from the stems of large specimens of Larix, 
Sequoia, and other conifers. 
The presence of perforations in the membranes is also indicated 
by the fact that large quantities of gases can be forced rapidly 
through the bordered pits of tracheids that are thoroughly saturated 
with sap. 
The surface tension of sap in the pit of membranes of various 
conifers can be overcome by pressures of less than 3 atmos- 
pheres. 
The tension hypothesis of the ascent of sap in plants, as inter- 
preted by Drxon, requires contirfuous columns of water, which, at 
the tops of trees, 75-300 ft. in height, are subjected to tensions 
of 5-20 atmospheres. In other words, the bordered pits must be 
impervious to undissolved gases under pressures of 5-20 atmos- 
pheres. 
It remains to be shown how continuous columns of water can 
be maintained in tall conifers when the membranes in the bordered 
pits may become permeable to undissolved gases at pressures of less_ 
than 3 atmospheres. 
In conclusion, the writer wishes to express his sincere thanks to 
various members of the United States Forest Service, Mr. R. D. 
SWALEs of the Union Lumber Company, and the Director of the 
