414 
IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
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The upper limit of tree growth is a transition area from the 
bare prairie to the wooded forest characterized by stunted trees 
and hardy shrubs able to endure almost as severe drought as the 
highly xerophytic plants cf the open prairie. 
A variation in conditions, due to exposure, sufficient to cause 
only a few degrees of difference in relative humidity, is enough 
to cause striking variation in forest covering, if the critical 
point for the plants struggling to find foothold is not passed. 
The variation in soil is not of itself to be charged with the 
variation in flora because the same soil carries all the different 
floras within a few hundred yards cf eacli oilier, the plants vary- 
ing according to the exposure. 
Department of Botany, 
State University. 
