THE ROLE OF WINTER TEMPERATURES IN DETER- 
MINING THE DISTRIBUTION OF PLANTS^ 
Forrest Shreve 
The investigation of the control of plant distribution by the 
various phases of the temperature factor is one of the most important 
tasks of physiological plant geography, at the same time that it is one 
of the most backward and difhcult. The field is an old one, the first 
outlines of which were sketched by Willdenow, Humboldt and Schouw. 
They delimited the great temperature zones of the earth, but in rela- 
tion to flora rather than to vegetation, — in relation to the distribution 
of species, genera and families, rather than to the great physiologically 
coherent assemblages of plants. To the investigations and explora- 
tions of these men and their immediate successors we owe the dictum 
(which is itself of later origin) that the character of the flora of a 
region is controlled by temperature, that of its vegetation by moisture. 
The only temperature datum used by the early plant geographers 
was the annual mean, and this is still used as the sole criterion in 
distributional studies by men who prefer that their generalizations 
should be broad rather than of scientific utility. The gigantic toil 
of the phenologists between 1850 and 1890 yielded some results on 
the operation of temperature, and gave us a vast accumulation of 
data of which some real use was made at the time, and to which we 
may return in future investigations. The men by whom this work 
was carried on were mostly climatologists, and their efforts were 
handicapped by the fact that they worked extensively rather than 
intensively, and that they had not a sufficient foundation of physio- 
logical facts upon which to operate. 
Our knowledge of temperature influences in the distribution of 
vegetation is fundamentally underlaid by our knowledge of tempera- 
ture influences on individual species of plants, and the two bodies of 
knowledge have come from fields of study which are widely unlike 
in their perspective and methods: from geography and from plant 
^ Paper read in the Symposium on "Temperature Effects" before the Botanical 
Society of America at Atlanta, December 31, 1913. 
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