WINTER TEMPERATURES AND DISTRIBUTION OF PLANTS 201 
about 2,000 ft. higher. The vertical limitvS of a large number of 
species have been found to differ by this amount as determined for 
canons and for ridges and slopes. 
It would appear, therefore, that the winter phases of the tem- 
perature factor sustain, in their distribution, a close relation to the 
distribution of plants. The experimental work carried out to test 
this correlation was planned only after a study of the relation existing 
between the vertical distribution of several plants throughout Arizona, 
and the vertical distribution of several phases of low temperature. 
Data were secured from a series of Weather Bureau stations extending 
from the low southwestern corner of Arizona to the cool highlands of 
the northern part of the state. The length of the frost season, the 
number of days with freezing temperature, the total number of hours 
(per winter) of below-freezing temperature, the greatest number of 
consecutive hours of frost, and the lowest minimum have all been 
ascertained, not in their average, but in their maximum intensity. 
The results indicated that the greatest number of consecutive hours 
of freezing temperature is the factor most closely corresponding, in its 
distribution, with the limitation of the species concerned. 
Experiments performed^ with succulent plants native to various 
altitudes in southern Arizona, indicated that the number of hours 
that they are exposed to temperatures below freezing determines 
their death, without regard to the absolute minimum reached during 
the freezing period (although minima below 1 8° F. were not used). 
The succulents which have the lowest vertical limit are unable to 
resist freezing over 19 to 22 hours in duration, while the species of 
higher and higher limits are progressively able to withstand longer 
and longer periods of freezing, up to 66 hours. With the limited 
hardiness of the Arizona species of cacti may be compared the behavior 
of Opuntia missouriensis, which withstood 375 consecutive hours of 
freezing temperature in the winter of 1910-11, at Havre, Montana, 
near the northernmost limit of the succulent type of plant. 
There are doubtless species other than those investigated in which 
the absolute minimum attained during freezing is of critical importance 
in causing death ; there are doubtless very low minima (never attained 
in Arizona) which would be fatal on very short exposure; and there 
are probably still other species of plants for which some of the other 
^ Shreve, Forrest. The Influence of Low Temperatures on the Distribution of 
the Giant Cactus. The Plant World 14: 136-146, 191 1. 
