WINTER TEMPERATURES AND DISTRIBUTION OF PLANTS 1 95 
physiology. The viewpoint of the geographer — and with him that 
of many floristic plant geographers — is too broad and general to give 
due regard to the actual physiological effects of temperature on plants ; 
the point of view of the plant physiologist, on the other hand, is often 
too intensive to enable him to realize that the "conditions" of his 
laboratory experiment are identical with the "physical factors" of 
the environment of plants growing under a state of nature, and he is 
therefore prone to neglect the bearing of his work on the problems of 
the field. There is no greater desideratum at the present time — with 
respect to the operation of all environmental factors — than to bring 
the intensive methods and exactness of logic which characterise 
physiological work to bear on the large and intricate problems of 
physiological plant geography. 
In a consideration of the influence of the various phases of the 
temperature factor on the limitation of the distribution of plants, and 
on their relative abundance in different parts of their areas, it will 
be seen that these phases fall into two well-marked groups. The 
first to be mentioned — and neither can be first in importance — 
are those phases of temperature which have to do with the length of 
the season in which growth and other activities are possible, with the 
curve of temperature conditions within this season, and with the 
possible effect of the highest portions of the seasonal curve as deterrent 
to the activities of plants. The second group of phases of the tem- 
perature factor are those which have to do with the length of the 
season during which low temperatures may exert a deterrent or fatal 
effect upon physiological activities, and with the duration and intensity 
of the critical periods in this season. 
Considerable physiological study has been given to the effect of 
gradients of temperature upon plant activities, and also to the effect 
of various durations and intensities of cold. Various attempts have 
been made to formulate the results of these physiological investigations 
in such a way as to give them general applicability in plant geography, 
and of these attempts it may be said in general that each has been an 
improvement upon its predecessors. It has been one of the most 
common errors of the phenologists that they have considered a degree 
at one part of the temperature scale the equivalent of a degree at 
another part of the scale. This impossible assumption has led to the 
totalling of daily mean temperatures as the means of securing an index 
of the possibilities for plant existence or plant growth in a given 
