have seen only one or two preserved specimens from this 
locality that showed signs of intergradation. 
Apparently, the differences in edaphic requirements 
of S. magnicamporum and the old field ecotype of S. 
cernua normally preclude their close association, but I 
have recently learned of a station that supports both 
taxa. The small number of specimens that I have seen 
from this locality do not allow definitive work, but it 
seems that the two species are maintaining their integri- 
ties at the site. The edaphic nature of the area that al- 
lows such an admixture is unfortunately unknown. 
Because of the several apparent isolating mechanisms 
present between S. cernua and S. magnicamporum and 
the evidently greatly restricted gene exchange, I con- 
sider them to be distinct species. 
Nevertheless, it is apparent that some gene exchange 
has occurred. Particularly in sand deposits, intermediate 
specimens may occasionally be found. In nearly all such 
‘cases of which [am aware, however, such plants are from 
disturbed sites; drainage, intensive pasturage, and ex- 
cavation are common factors in Illinois. Fewer interme- 
diates are found in areas where the S. cernua influence 
must have been contributed by the old field ecotype, and 
only limited introgression is indicated. 
One major intermediate race is known, however, and 
it complicates the situation. In northern Illinois, there 
occur, in low black-soil prairie, populations somewhat 
intermediate between S. magnicamporum and the sand 
prairie S. cernua. It appears that rare crosses between 
these taxa have produced a race that occupies a hybrid 
habitat. This race differs from typical S. magnicamporum 
in the presence of a basal dilation of the lip and the poor 
development or even complete absence of the central 
thickening. I have seen only two live plants of this taxon 
in bloom, and these were apogamous. Although scented 
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