128 Drs. H. T. Brown and W. E. Wilson. [Jan. 9, 
temperature and degree of movement of the air, then the difference in the 
amount of water transpired in a given time as determined by the balance 
will, when multiplied by the heat of vaporisation, give a measure of the excess 
number of calories entering the cooler leaf for a temperature gradient equal 
to the temperature difference given by the integrated record of the drum during 
the time occupied by the experiment. 
In order to produce the differential transpiration on which the success of 
such experiments depends, advantage is taken of the fact that the stomata, 
through which the greater part of the vaporisation of the water takes place, 
are very seldom equally distributed on the two sides of the leaf. It is only 
necessary, therefore, to arrange one pair of leaves with their dorsal sides 
turned towards the platinum coils, and the other pair with their dorsal sides 
facing outwards. Hypostomatous leaves lend themselves best to such 
experiments, providing they are by nature good transpirers, since the dorsal 
side with its imperforate cuticle loses relatively little water compared with 
the ventral side. 
Any changes in the dimensions of the stomata during an experiment have 
no influence on the final result, owing to the comparatively rapid thermal 
adjustments which take place in the leaf. Mr. Francis Darwin has already 
recorded the fact* which our experiments fully verify, that a slow and gradual 
closure of the stomata takes place when a cut leaf is placed in darkness. 
As the stomata close the record of differential temperature on the drum of the 
recorder approaches more closely the zero line of no temperature difference, 
but it is the mean value with which we alone have to deal in these experiments 
and, providing the record is correctly integrated, this mean temperature 
difference and the loss in weight of the two pairs of leaves respectively are 
the only experimental data required for determining the thermal emissivity of 
the leaf surface in absolute units.T 
As an example of such an experiment performed under “still air” 
conditions, we may give the following. The leaf cases were placed in a 
covered tin case which was enclosed in an outer wooden case. A _ tray 
containing calcium chloride was placed at the bottom of the tin case in order 
to keep the air sufficiently dry to promote transpiration. 
Experiment on the (hypostomatous) leaves of Liriodendron tuliprfera. 
Temperature of air in the enclosure 18°6 C. 
No. 1 Coil.—Leaves arranged with stomatiferous surfaces outwards. 
* Loe. cit. 
+ Before each set of experiments it is always necessary to verify the zero line on the recording 
drum, since this is at times subject to small variations which are difficult to explain. 
