36 
JOHN H. EHLERS 
not read directly by means of the galvanometer, but by a zero method 
which was to adjust the temperature of the bath containing the 
control junction until the galvanometer showed no deflections. The 
temperature of the bath then indicated the leaf temperature. This 
method was easily accurate to within .5° C, which was considered 
sufficient for the purpose. 
Under strong illumination from the powerful Keith burner, leaves 
of cherry laurel reached a temperature of 10° C. or more above the 
temperature of the bath. 
The second of the above mentioned papers was presented before 
the Royal Society of London in March, 1905, by Brown and Escombe 
(4). The paper deals with investigations on the physiological pro- 
cesses of green leaves. It is interesting in this connection because of 
the method employed to determine leaf temperatures. The tempera- 
tures were arrived at by complicated calculations involving (i) the 
coefficient of absorption of radiation, (2) the specific heat of the leaf, 
(3) the energy expended in photosynthesis and respiration, (4) the 
thermal emissivity of the leaf, and (5) the effect of the wind velocity. 
Both attached and detached leaves of Helianthus annuus and Senecio 
grandifolms were used. Under full solar illumination, a maximum 
temperature difference between leaf and air of only 2° C. was found by 
this method. This work was done at air temperatures of 15° to 27° C. 
In strong contrast to the results of Brown and Escombe are those 
obtained by Blackman and Matthaei (2) in their investigations of 
vegetable assimilation and respiration. Determinations of leaf 
temperatures under natural illumination, both with the leaf in open 
air and enclosed in a glass case, were made thermo-electrically. The 
apparatus and method used were identical with those used by Matthaei 
(14). For experiments in the open air detached leaves of cherry laurel 
were stretched on a frame, the leaf stalk dipping in a well of water. 
The thermo- junction was inserted in the midrib. Under brilliant 
insolation, temperatures varying from 7° to 16° C. above the shade 
temperature of the air were obtained. In diffuse light the leaves were 
found to be from 1° to 3° C. above the air temperature. Of the two 
methods — the complicated calculations of Brown and Escombe and 
the thermo-electric method of Blackman and Matthaei — the latter 
seems by far the more trustworthy. The sources of error are very 
much reduced. 
In confirmation of the results obtained by Blackman and Matthaei 
