io8 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 diflicult. Potentilla possesses 

 a copiogregarious or a solitary distribution along the middle and 

 lower slopes. It rises 50 to 70''"^ 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 is a 

 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 marginaia, 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 ter- 

 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 90'^"^ 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- 

 nal spikes of dark-blue flowers make it a very striking object, especially 

 when it occurs in such abundance. With sod-establishment it gradu- 

 ally disappears, being a characteristic component of the bunch-grass 

 stage. The indehiscent one-seeded pod is highly immobile. 

 The University of Chicago 



