A Convenient Modification of the Porometer. 213 
attached to the leaf, so that the air drawn through the leaf bubbles 
through the water. Any required pressure may he obtained by 
adjusting the length of the out-flow tube, which dips into a 
small overflow vessel, or, alternatively, the water may be allowed 
to drip from the end, though in the latter case there are slight 
pressure changes due to the formation and discharge of the 
succession of drops. 
It has been found convenient to insert a three-way stopcock 
between the leaf chamber and the aspirator bottle in order to place 
the chamber in connexion with the outer air, and so release the 
pressure upon the leaf, when no reading is being made. 
The time elapsing between the discharge of two consecutive 
bubbles from the air-intake tube is taken as an indication of the 
size of the stomatal apertures, and this period can be determined 
with considerable accuracy by means of a stopwatch. 
The form of the apparatus is shown in the accompanying 
diagram. 
Compared with the Darwin and Pertz form, this aspirator 
method shows to advantage, and at the same time presents but few 
difficulties. 
1. The pressure drawing the air through is constant, except 
for very slight variations as each bubble forms, and even when 
these variations have the greatest effect, i.e., when the pressure 
difference employed is small, the cycle will be the same for every 
bubble and therefore will not detract from the accuracy of 
measurement. 
2. The need for raising a water or mercury column for each 
separate reading is eliminated and this is a great convenience when 
several consecutive readings have to be made. 
3. With any particular setting of pressure, area of leaf 
chamber and size of bubble, the method is applicable to a wide 
range of stomatal changes, since, if the rate of air-flow increases 
sufficiently to render impossible the accurate timing of one bubble, 
any number may be included without inconvenience and an average 
obtained. Using the Darwin and Pertz method, the same 
contingency may be met either by altering the average pressure, 
thereby necessitating a calculation, or by maintaining the same 
average pressure and altering the distance through which the 
column falls. In either case there is a possibility of introducing an 
error into the reading. When the pressure inside the leaf chamber 
is reduced in ol der to draw air through the leaf, there is a natural 
