THE TEMPERATURE OF LEAVES OF PINUS IN WINTER 33 
and its possible bearing upon photosynthesis and the accumulation of 
reserve food material during the same season, that this investigation 
was undertaken. The work was carried on in the Botanical Depart- 
ment of the University of Michigan during the winter months of 
1912-1913 and 1913-1914. The problem was suggested to the author 
by Professor F. C. Newcombe, and was carried to completion under 
his supervision. To him my sincere thanks are due for helpful sug- 
gestions and kindly criticism. Grateful acknowledgment is also made 
to Professor H. M. Randall of the department of physics for valuable 
suggestions and help in vsetting up the apparatus, and for apparatus 
loaned by him; and to Professor N. H. Williams of the department 
of physics for the use of his laboratory and for helpful advice given. 
II. Historical 
The papers of particular interest for the consideration of the 
problem under discussion may be grouped under three heads: (i) the 
internal temperature of foliage leaves; (2) photosynthesis at low 
temperatures; (3) accumulation of reserve food material by evergreen 
trees in winter. 
I . The Internal Temperature of Foliage Leaves 
Of interest here, merely because it is the first record of an investi- 
gation to determine leaf temperatures that could be found in botanical 
literature, is a paper by Rameaux (21) published in 1843. This in- 
vestigator placed neighboring leaves, attached to the stems, one 
upon another until the layer was sufficiently thick to prevent any 
light from passing through, and then bent the layer around a mercury 
thermometer. No record of leaf temperatures is given, however, 
since the investigation was chiefly concerned with finding the tempera- 
ture of stems. 
Schumacher (27) was the next to attempt a determination of leaf 
temperatures. His method consisted in placing a thermometer 
against the lower side of a leaf exposed to solar illumination. At 
least two serious objections may be raised to this method: (i) Assum- 
ing a difference in temperature between the leaf and the surrounding 
air, only the small portion of the mercury bulb in contact with the 
leaf would be influenced by the leaf temperature; (2) the rays passing 
through the leaf would tend to warm the mercury regardless of the 
leaf temperature. 
