66 
Knight.— On the Use of the 
Readings are again taken, and the differences between these and the previous 
ones are an indication of the added resistance due to the extra length of 
intercellular spaces through which the air has now to pass. 
This method is easier of manipulation than the vaselining one, in that 
the extra length of tissue can be introduced at will and without any shock 
to the plant, an important consideration in view of the effect of shock upon 
the stomata already described. 
In constructing the double chamber it is advisable to ensure that 
the area of leaf between the outer and inner chambers (i. e. DD in Fig.) is 
somewhat greater than the area covered by the inner chamber (h), so that, 
when the outer chamber is open, the conditions may closely approximate 
to the ordinary single-chamber experiment. If DD is greater than H, 
there will be less tendency for air to enter the leaf at EE when C is 
open. 
The usual method of procedure was to take three readings in succession 
with the outer chamber open, the mean of these representing the stomatal 
resistance plus the resistance of a short length of intercellular spaces. The 
outer chamber was then closed and readings taken continuously till they 
became constant. Finally, the outer chamber was again opened and three 
more readings taken, in order to allow for stomatal changes during the 
experiment, the mean of the first and last three being considered to be the 
correct c open ’ reading. 
It was found that readings taken immediately after closing the outer 
chamber were not always the same as those taken a few minutes later, and 
this was attributed to the fact that at the moment of closing the outer 
chamber the air in it was at atmospheric pressure, but before any equilibrium 
could be set up the pressure in the outer chamber must approach that in the 
inner one. 
The difference between the ‘ open ’ and ‘ closed * readings was expressed 
as a percentage of the ‘ open ’ reading, this latter being regarded as the 
closer approximation to the indication of stomatal resistance. For each leaf 
investigated, several of these composite readings were taken at different times 
of the day, corresponding to different dimensions of stomatal aperture, and 
the series of percentages thus obtained were plotted as ordinates with the 
reciprocals of the ‘open’ readings as abscissae, i. e., roughly, the effect 
of the added length of intercellular spaces was plotted against stomatal 
aperture. 
It must be noted here that this method of plotting is entirely arbitrary, 
and the figures obtained only refer to one particular set of dimensions of the 
chambers used, but by expressing the results in this manner the conclusions 
drawn from them are easily demonstrated. 
Experiments with double chambers have been carried out with a 
variety of plants, including Hedera helix , Eucharis amazonica , E. Mastersi f 
