THE TEMPERATURE OF LEAVES OF PINUS IN WINTER 
37 
are those obtained by Smith (28) in Ceylon. The apparatus and 
methods employed by Smith were practically the same as those used 
by Blackman and Matthaei and, therefore, need no further description. 
His results may be summarized in part as follows: (i) Thermo-j unctions 
give the same result whether inserted in the lamina or in 'the midrib 
of the leaf ; (2) "in still air, with the black bulb in vacuum thermometer 
at from 55° to 62° C, the air temperature in the shade being 25° to 
28° C, leaves, whether thick and fleshy, or thin and pliable, when 
placed normal to the sun's rays, may reach a temperature of 15° C. 
above that of the surrounding air;" (3) "in the shade such leaves 
have an internal temperature varying from 1.5° C. below to 4° C. 
above that of the surrounding air;" (4) thickness and texture of leaf 
have little effect, but thick leaves require more time to reach the 
final temperature and cool off more slowly; (5) air currents and 
transpiration are important factors in reducing leaf temperatures. 
2. Photosynthesis at Low Temperatures 
While it has long been recognized that photosynthesis is influenced 
by the temperature to which the plant is exposed, comparatively 
little work has been done to determine the relationship between them. 
This is especially true of photosynthesis at low temperatures, such as 
prevail during the winter season in cold temperate regions. Until 
within recent years only a few more or less isolated determinations 
of the lowest temperature at which photosynthesis begins have been 
made. 
Among the first investigators of photosynthesis at low temperatures 
was Boussingault (3) who observed an evolution of oxygen by Pinus 
laricio at a temperature of 0.5° to 2.5° C, and by meadow grasses at 
1-5° to 3.5° C. It is interesting to note that Boussingault took into 
account the effect of solar radiation upon the leaf temperature. The 
plants with which he experimented were placed on the north side of a 
wall to protect them from the sun's rays during the experiment. 
Heinrich (8) found that in Hottonia palustris carbon-dioxide assimi- 
lation began at 2.2° R., while Kreusler (11), experimenting with 
Phaseolus vulgaris, Ricinus communis, Prunus Laurocerasus, and 
Rubus, observed an assimilation of CO2 at temperatures of — 0.9°, 
^ 0.6°, — 2.2°, and — 2.4° C. respectively. 
Jumelle (9), as a result of experiments with Evernia Prunastri, 
Picea excelsa, and Juniperus communis, makes the assertion that 
