32 UNIVERSITY OF MISSOURI STUDIES [^74 



munity is most frequent in flats which are overflowed in spring 

 and early summer. The grasses and sedges are thus kept out, 

 against which these herbs cannot compete. The smartweeds 

 seize also low cultivated and waste grounds, such as cornfields 

 which have suffered from floods. This community infests 

 muddy shores, sandy stream-flats, and even waste places about 

 barnyards and houses. The following are the prevailing spe- 

 cies: 



Polygonum hydro- P. Pennsylvanicum 



piperoides P. lapathifolium 



P. incamatum P. nodosum 



P. acre P. Persicaria 



P. Hydropiper 



e. Uliginosae coreopsidoides. The tickseed associa- 

 tion covers acre upon acre of lowlands, transforming these into 

 seas of gold in autumn. This is the flora proper to low prairie 

 flats. The aestival vegetation consists mainly of various Labia- 

 tae, Verbenaceae and Scrophiilariaceae (Stachys, Lycopus, 

 Mentha, Blephilia, Verbena, Lippia, Mimulus, Scrophnlaria, 

 Veronica, etc.). The autumnal vegetation consists prevailingly 

 of Compositae (Eupatorium, Silphium, Rudbeckia, Helian- 

 thus). Among non-composites Gerardia Besseyana and Lobelia 

 syphilitica are conspicuous. Perhaps no other association con- 

 sists so wholly of Gamopetalae as this. The typical members of 

 this group with forked achenes or hooked burs are : 



Xanthium Pennsylvanicum Bidens frondosa 



X. speciosum B. connata 



X. commune B. comosa 



X. glabratum B. cernua 



Coreopsis involucrata B. bipinnata 



C. discoidea 



