106 BOTANICAL GAZETTE | AUGUST 
feet. The summit of the Staked plains above 4000 feet has, as. 
regards temperature, the climate of the southern half of Kansas, 
Illinois, and Missouri; but on-account of low average rainfall 
and prevailing high southwest winds it has the characteristics of 
the high plains climate of western Kansas and Nebraska. In 
trans-Pecos Texas the Upper Sonoran zone is a series of islands 
represented by the isolated mountain summits rising out of the 
Lower Sonoran zone. 
TRANSITION ZONE.— The highest peaks of the Guadalupe and 
Davis mountains possess a considerable number of transition. 
herbaceous and shrubby species ranging north in the mountains. 
of Montana and Washington, beside forest tracts of Pinus ponde- 
rosa, P. flexilis, and Pseudotsuga taxifolia, which are typical transi- 
tion forests. 
FLUCTUATIONS IN ZONAL BOUNDARIES.— If we accept the law 
of temperature control “that the distribution of boreal species: 
southward is limited by the mean temperature of a few weeks: 
during the hottest part of the summer,” those annuals which 
begin their activity at a relatively low temperature, and complete 
their vegetative and fruiting period before the approach of 
summer temperature, would enjoy a distribution far within the 
Austral regions. To state the case differently, within the Lower 
Sonoran or semi-tropical zone the lower temperature prevailing” 
during February, March, and April, in which certain boreal. 
species complete their period of fruition, would be for such. 
species as truly a boreal zone as if they completed their fruition 
period during April, May, and June in correspondingly higher 
latitudes. This fact can account for the presence of such species 
as Evax prolifera, Draba cuneifolia, Anemone Caroliniana, Corydalis 
aurea, Linaria Canadensis, and others, within the borders of the 
semi-tropical zone. These all come into flower in the last half 
of February. This also explains how many prairie annuals,. 
which are so abundant in their season as to give the dominant. 
tone to the vegetation, may appear in this way successively over 
the whole Great plains area and south to Brownsville, or more: 
frequently over the central prairies of Kansas, Missouri, and- 
