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ECOLOGICAL CROSS SECTION OF THE MISSISSIPPI. 



173 



Hydrastis canadensis ^ Ranunculus ahortivus, R, septentrionalis , 

 Podophyllum peltatum^ Capsella Bursa-pastorisy Claytonia 

 virginicaj Geranium maculatiimj Oxalis strictay Callitriche 

 deflexa Austini, Circaea Luietianaj ChaerophijTlum procumbens, 

 Sanicula marylandica canadensis, Scnecio aureus. Phlox di- 

 varicata, Ellisia Nyctelea, Monarda fistulosa, Nepeta Glechoma, 

 Erythronium alhidum, Trillium recurvatum and T. sessile, 

 Uvularia perfoliata, Arisaema Dracontium and A, triphyllum, 

 two or three species of Carex, Elynius striatus and Poa pra- 

 tends, and numerous excellent specimens of Adiantum 



Cystopteris fragil 



the 



they become less striking owing to the appearance of 



numerous other species, especially of 

 biatae- Among the former are 



d La- 



Actinomeris 



\eris, Eupatorium purpureum, Helenium nudifloi 

 • laciniata and Xanthium canadense. Striking 



s 



and 



become 



form an almost im 



bring any marked changes on the 



Autumn does not 

 : bankS; very few 



bein 



Prairies . 



The prairies which cover limited areas of the Missouri 

 plateau owe the peculiar character of their vegetation to a 

 combination of geological, physiographical and meteorological 

 conditions. The practically horizontal stratification of the 

 St. Louis limestone, which here is covered by loess, and the 



to 



but 



amounts of precipitation during the earlier part of the year 

 when the amount of evaporation is slight. When finally 

 this moisture seeps away through the fissures in the Hme- 

 stone, it is permanently lost. In contrast to the almost 

 swampy conditions of the spring, the soil, during summer 

 and autumn, is dry, the evaporation being largely in excess 

 of the precipitation and the underground drainage almost 

 nil. Hence the spring brings a luxurious growth of grasses 

 and is the onlv Deriod when sufficient moisture is present for 



