THE AMERICAN BOTANIST. 
I I I 
to a region which required at least 10,000 degrees, the plants of 
which were apparently unable to withstand six consecutive weeks 
of daily summer temperature averaging 71.6 degrees, and the 
Canadian, Hudsonian, and arctic extending thence northward into 
the polar region, their southern limits being marked by the isoth- 
erms for the six consecutive hottest weeks of 64.4 degrees, 57.2 
degrees and 50 degrees, respectively. The conclusion of the sur- 
vey in regard to the distribution of plants and animals is as fol- 
lows : ''The northward distribution of terrestial animals and 
plants is governed by the sum of the positive temperatures for the 
entire season of growth and reproduction, and the southward dis- 
tribution is governed by the njean temperature of a brief period 
during the hottest part of the year." 
Vegetation throughout the greater part of the United States 
lies within the temperate (or austral) zone, and is dormant during 
several months of the year. It is within this region, however, 
that our largest staple food crops are produced. Germination 
and growth are wholly dependent upon the temperature in excess 
or deficiency of 43 degrees. Adamson says that "the develop- 
ment of buds is determined by the sum of the daily temperatures 
counted from the beginning of the year." But it is likely that a 
more accurate expression would be the sum of the daily tempera- 
tures from the cessation of winter. 
The adaptability of plants to climate is so great that it is ques- 
tionable if the heat units obtained for one latitude would be those 
required in a more northerly or southerly lahtude. Take wheat, 
for instance; it can be grown successfully within the semitropical 
zone and thence northward beyond the cold temperate zone, a 
range of probably 20 to 25 degrees of latitude, and in the northern 
limit the rate of growth is the more rapid. Hoffman, in a series 
of experiments at Giessen, Germany and Upsala, Sweden, found 
that the time of the blooming of the Syringa vulgaris (common 
lilac) was April 29 at Giessen and June 17 at Upsala. The heat 
units received were 1,482 degrees and 1,433 degrees C., respective" 
ly, practically the same. In the first ripening of fruits, 1 1 species 
being considered, the ratio between the two places was as 100 to 
82, while for the interval between the first bloom and the ripening 
