THE TEMPERATURE OF LEAVES OF PINUS IN WINTER 67 
by an experiment in which the leaf was partially protected against 
air currents. Under these conditions a differential temperature of 
10.31° C. was obtained (Table XIV). Under less brilHant illumination 
and stronger air currents the differential temperature is correspond- 
ingly less. Even diffuse light, according to its brightness, will increase 
the leaf temperature from 0.5° to 2° C. For February, the coldest 
month of the year, the average differential temperature between 
leaf and air — 650 readings in all, taken between the hours of 8 a.m. 
and 3 p.m. and under all kinds of weather conditions, cloudy days as 
well as days of sunshine — was 3.06° C. 
The differential temperatures in the winter season, as was to be 
expected, are considerably less than those obtained by previous 
investigators for broad leaves under summer and tropical insolation, 
namely, from 7° to 16° C. The cause of the difference is to be sought 
in the lower rate of respiratory changes (Molisch (19)), the lower 
intensity of solar radiation, and in the greater loss of heat by con- 
vection, air currents being, as a rule, more constant and stronger during 
the winter season. Though smaller in winter, the differences are, 
nevertheless, of sufiicient magnitude to become an important factor 
in photosynthesis. 
It should also be remembered that the differential temperatures 
as found in this investigation are those of the leaf as a whole, and not 
necessarily those of the chloroplasts. It is the chloroplasts that 
absorb the most of the radiant energy; the temperature of the leaf is 
due to radiation and conduction from the chloroplasts. It is entirely 
probable, therefore, that the chloroplasts have a temperature con- 
siderably higher than that of the leaf as a whole. 
Recalling briefly the more important facts of the evidence at our 
disposal concerning photosynthesis at low temperatures, we have: 
(i) Evidence of an accumulation of reserve food material through 
the winter in trees with persistent leaves as found by Sablon; (2) con- 
firmatory evidence both of photosynthesis and of an accumulation of 
reserve food in winter by evergreen leaves as found by Miyake; 
(3) evidence of photosynthesis at — 6° C. as the result of very careful 
work by Matthaei; (4) the finding of Ewart that a number of ever- 
green shrubs and trees including conifers, after having been exposed 
for some time to a temperature often falling below — 15° C. had no 
power of assimilation when tested at 1° C. 
The results obtained by Ewart appear to contradict those of 
