2 I 2 
R. C. Knight. 
A CONVENIENT MODIFICATION OF THE POROMETER. 
By R. C. Knight, B.Sc. 
[With One Figure in the Text]. 
I N the Darwin and Pertz form of the porometer 1 air is drawn 
through the leaf by means of a column of water, the rate of 
flow of the air being measured by the rate of fall of this column, 
and this rate is taken as an indication of the size of the stomatal 
pores. 
A column of mercury in a narrow-bore horizontal tube with a 
short vertical arm at one end, dipping into a reservoir of mercury, 
can be used instead of the vertical column of water, thus obviating 
the change of pressure which occurs as the water falls, but in 
both these forms the water or mercury needs to be raised to its 
original position after every reading. 
After working for a short time with the above-mentioned types, 
a device was adopted whicli proved itself more convenient than 
either, and which is also free from many of their drawbacks. 
The pressure difference required to draw air through the leaf 
is obtained by means of a head of water in an aspirator bottle, and 
this pressure difference is kept constant throughout the experiment 
by the usual expedient of keeping below the surface of the water the 
opening of the intake tube, with which is connected the chamber 
1 Proc. Roy. Soc., 84. B., 1911, p. 136. 
