A Convenient Modification of the Porometer. 215 
at 37‘9°C. gave bubbles containing 0*115 c.c., a decrease of nearly 
9%. This temperature range is, however, far greater than need be 
experienced in actual work. If the aspirator is shaded from direct 
sunlight the temperature change need never be more than 5°C., 
the effect of which is negligible. 
5. Temperature changes are obviously liable to interfere with 
the system in ways other than that described above. A rise of 
temperature would cause an expansion of the air already in the 
aspirator, and although this would be almost immediately compen¬ 
sated by the exit of water, it is conceivable that there might be 
some slight temporary effect upon the rate of bubbling. Sudden 
temperature changes should therefore be avoided, such as might 
arise from filling the aspirator from the tap and immediately 
beginning an experiment in a warm laboratory. Ordinary tempera¬ 
ture changes in a laboratory or greenhouse are so gradual as to be 
negligible, especially as each reading need occupy only a few 
seconds. If greater accuracy is required the aspirator may be 
placed in a water bath. 
A description of a typical experiment will perhaps best serve to 
indicate the details of the apparatus and the method of use. 
Exp. 56 ; December 16th, 1914. 
The volume of the aspirator bottle was nearly two litres, the 
air-intake tube at which the bubbles formed, 4 mms. internal 
diameter, and the pressure difference 13 cms. of water. The plant 
used was Euchnris Mastersi, the stomata of which are capable of 
opening very widely under suitable conditions, and occur only on 
the under surface of the leaf. The leaf chamber enclosed an area 
of about 1 sq. cm. and under these conditions, the day being bright 
with cloudy intervals, and the plant in a greenhouse at about 
20"C., at 10.30 a.m. the time taken for one bubble to form and 
discharge was 2-9 seconds, five bubbles being actually timed and an 
average obtained from these. By 12.10 p.m. the time for one 
bubble had decreased to 1*8 seconds (average of 10), whilst the last 
reading, taken at 4.10 p.m., twenty minutes after sunset, was 21*2 
seconds for one bubble, with the stomata closing rapidly. 
As deduced from these rates of bubbling, the apertures of the 
stomata in this experiment, as has been indicated, were rather 
larger than the average, but higher rates are sometimes found, 
especially with plants the leaves of which have stomata upon both 
surfaces. Helianthus tuberosus on October 1st, 1914, a very bright 
hot day, with apparatus similar to that in the experiment already 
