May 19.1923 
Statistical Study of Biologic Forms 
561 
PHYSICAL FACTORS 
Unfavorable cultural conditions affect the virulence and spore size of 
the rust fungus to an appreciable extent, especially the quantity of rust 
produced and the length of the spores. The inherent nature of i^ection 
and the width of the spores are rarely and only slightly changed, 11 at all. 
For obvious reasons the effect of environment on the uredinial stage 
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Fiz. lo.-Differences in widths of aeciospores, uredinipspores, and teliosporM of biologic forms of Puccinia 
graminis grown on susceptible and resistant host plants. 
alone was studied. Excessive heat more effectively and more con¬ 
clusively caused the urediniospores of the wheat rust to become shorter 
than did very low temperature. Deficiency in sunlight and in soil 
moisture appeared to be equally influential in diminishing the size of the 
urediniospores. In all of these cases as well as in the control experiment 
