196 
FORREST SHREVE 
locality. Thanks to the growing interest manifested in this subject 
by men with physiological training, the crudities of the earlier work 
are being eliminated from the most recent utterances on this subject. 
One of the most widely used schemes for the formulation of tem- 
perature data in such manner as to give them general applicaoility 
in biogeography, is the system of life zones proposed by Merriam for 
North America. These zones are based on the isothermal lines which 
indicate the totalled degrees of temperature for the growing season. 
Not only does the placing of such lines rest fundamentally upon the 
assumption that every degree of temperature is the equivalent of 
every other degree in respect of plant activity, but it takes no account 
whatever of the second phase of temperature effects which are exerted 
in the frost season. 
Not only is Merriam's scheme of life zones to be criticized as a 
geographical delineation of temperature conditions, but it is even 
more fundamentally faulty when it is urged as a general scheme of 
classification of biogeographical regions. In spite of the importance 
of temperature as a factor in distribution it is illogical to take it as the 
sole criterion for the limits of distributional regions, especially when 
the role of soil and atmospheric moisture is so obviously of vital im- 
portance and is so potent in determining the areas of the principal 
vegecational regions of the globe. 
The temperature phases of the growing season and those of the 
frost season are in .no respect reciprocal or complementary to each 
other, excepting in the mere matter of the length of the two seasons. 
The end effects of the temperature conditions of summer and those of 
winter are quite distinct as they are registered in the limitation of 
the range of species. In the case of plants which range over large 
areas it is quite commonly the winter phases of temperature which 
limit their northward distribution, and the summer phases which 
restrict them at their southern edge. In many other cases it is the 
summer phases which determine the northern limit of the conditions 
that make it possible for a species to grow. 
More attention has been given by phenologists to the temperature 
phases of the growing season, and their potentialities, than to those 
of the frost season. The writer has welcomed, therefore, an oppor- 
tunity to make some preliminary investigations of the importance of 
the temperature phases of the frost season in determining the distribu- 
tional limits of some sub- tropical desert plants. 
