108 BOTANICAL GAZETTE [AUGUST 
importance except Acerates, reaching their maximum display in the 
serotinal aspect. Acerates appears rarely and as a solitary xerophyte — 
upon the higher slopes and along crests. It is a perennial with 
numerous permobile comose seeds, yet its abundance is always low. 
Its solitary umbel of greenish flowers, which blend with the foliage 
of the prairie, renders its detection difficult. Potentilla possesses 
a copiogregarious or a solitary distribution along the middle an 
lower slopes. It rises 50 to 7o°™, with the erect stems terminating 
in loose cymes of numerous yellow flowers. It thus exercises a local 
effect in the floral aspect. Through its numerous annual rosettes it 
also contributes in a limited degree to the plant covering. It isa 
perennial from a thick root. The numerous achenes are highly 
immobile, resulting in a limited distribution and the gregarious habit. 
The white-margined spurge, Euphorbia marginata, is an annual which 
occurs subcopiously on lower slopes and rises erect to a height of 50 
to 75°". The stems bear abundant bright-green leaves and are tet- 
minated by three-rayed umbels whose greenish-white flowers are sub- 
tended by involucres of numerous white-margined bracts, making the 
entire umbel a very conspicuous object. Amorpha canescens is a 
prominent perennial shrub (50 to go°™ high) exerting a controlling 
influence. It is a marked xerophyte and may rise to primary rank | 
upon the crests in a sub-copiogregarious distribution, but rarely occurs — 
upon the lower mesophytic slopes. Its gregarious habit and its abun- 
dant and densely white canescent leaflets and densely clustered termi- F 
nal spikes of dark-blue flowers make it a very striking object, especialy 
when it occurs in such abundance. With sod-establishment it 
ually disappears, being a characteristic component of the bunch-grass 
stage. The indehiscent one-seeded pod is highly immobile. 
THE UNIVERsITY oF CHICAGO 
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