BOTANY: H. H. M. BOWMAN 
685 
The advantages of this type of manometer are readily apparent. 
Its range compared to that of other high vacuum gauges is very large, 
extending from more than 10"^ mm. to as low pressures as can be ob- 
tained, without any change of apparatus. On account of its simplicity 
of construction it is inexpensive and exactly reproducible. Since there 
are no moving parts there are no difhculties due to vibration. The 
pressures of vapors which would not be registered on the McLeod gauge 
are measured by the ionization manometer. One of the greatest ad- 
vantages is the rapidity and ease with which measurements of a vary- 
ing pressure may be made since only the reading of a galvanometer 
need be followed. 
Many applications for which other manometers cannot readily be 
used at once suggest themselves, such as the measurement of vapor 
pressures of metals, etc. Since the device may be made with extremely 
small volume the pressure of very small quantities of gas may be meas- 
ured. It would also be useful to measure pressure changes over a long 
period of time for which more expensive manometers could not well 
be employed. 
A number of interesting physical measurements other than the meas- 
urement of pressure can be made with devices operating on the principle 
of this manometer, among which is that of the removal of occluded 
gases by electron bombardment. It is also hoped that experiments 
with various gases will give some information as to the relative cross 
sectional areas which different kinds of molecules present to the elec- 
tron discharge, for although the constant of the manometer was found 
approximately the same for hydrogen, air, and mercury vapor, more 
exact measurements might show differences due to different molecular 
diameters. 
PHYSIOLOGICAL STUDIES ON RHIZOPHORA 
By Howard H. M. Bowman 
DEPARTMENT OF BOTANY, UNIVERSITY OF PENNSYLVANIA. AND TORTUGAS 
LABORATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Received by the Academy, November 10, 1916 
Research on the physiology and ecology of the red mangrove, Rhizo- 
phora mangle, which has engaged my attention for the past few years, 
was continued at the Tortugas Laboratory of the Carnegie Institution 
during the summer of 1916. The phase of physiologic investigation 
most emphasized was that relating to the transpiration rate of Rhizo- 
phora seedlings grown in solutions of different concentrations of salt 
