THE TEMPERATURE OF LEAVES OF PINUS IN WINTER 39 
With this method the lowest temperatures at which determinations 
can be made is approximately i° C. Parts of plants that were grow- 
ing in the open and had been exposed for several weeks to temperatures 
often reaching — 15° C. were thawed and examined at 1° C. The 
following plants among others, although most of their leaves remained 
alive, showed no power of assimilation at 1° C. : Ilex Aquifolitim, Buxus 
se?npervivens var. arborescens, Pinus Montana, Taxus baccata, Thuya 
occidentalism and Juniperus Sabina. Experiments were also made 
with tropical, subtropical and water plants. His results are sum- 
marized as follows: "It appears that all evolution of oxygen ceases in 
tropical plants between 4° C. and 8° C; in warm temperate, sub- 
tropical and water plants between 0° C. and 2° C, whilst in cool 
temperate, arctic, and alpine plants assimilation only ceases when 
the plants are frozen, i. e., at a few degrees below 0° C." The last 
case is, presumably, an inference, since he offers no experimental 
proof to substantiate it. He ascribes the cessation of assimilation in 
cool temperate, arctic, and alpine plants to physical causes, i. e., the 
withdrawal of water from the protoplasm to form ice crystals and the 
consequent desiccated condition of the tissue. 
The most important contribution to the question under considera- 
tion was made by Matthaei (14). Determinations of the rate of 
assimilation through a range of temperatures from — 6° C. to 45° C. 
were made. The leaves of cherry laurel, Prunus Laurocerasus var. 
rotundifolia, were used for this purpose. Her work differs in two 
respects from that of all previous investigators: (i) In that careful 
attention was given to keeping the material under uniform conditions 
before the experiment; (2) in that precautions were adopted for 
obtaining the true internal temperature of the leaf. The leaf temper- 
atures obtained and the method of determining them have already 
been briefly stated. Of the results obtained those which are of 
particular interest here are : 
1. The minimum at which assimilation could be observed. This 
was — 6° C. Disregarding Jumelle's finding as untrustworthy, this 
is the lowest temperature at which photosynthesis has been observed. 
The author further suggests that, for such cold-enduring plants as the 
conifers, assimilation may take place at temperatures considerably 
lower. 
2. The assimilation curve. This curve shows the maximal amount 
of assimilation at all temperatures from — 6° C. to 43° C. From 
