h 



256 MISSOURI BOTANICAL GARDEN-. 



American Bottom. All stages are represented at the mouth 

 of the River des Pores in what is known as Horn's Grovo 



(plates 



the soil has been 



an unimportant factor. Willows form the first woody vege- 



. islands and shores _ ..._....^,.., 



as they do on the clayey bottoms or on the swarapv uplands 

 (plate 14, 1). "^^ ^ 



But have we a right to assume that the first beginning of 

 plant growth in this region took place under the climatic 

 condHions which prevail to-day? Perhaps the vegetation 



the retreat of the ice field. It is but reasonable 

 to suppose, judging from conditions such 



■ \.^ 



as 



I 



bored 



that the 



lowed by willows with a temporarv climax of 



d 



Evidence is lacking that any of the latter have 



but the lack of such 



The conditions necessary for petrifaction were absent. Even 

 had they existed, all remnants of an earlier flora would 

 have been removed by the subsequent action of the waters 



shown by 



towards the end 



The existence of a 



there 



between it and the flora of to-day. Between the two lies the 

 most recent period of submergence. Judging from the thick- 

 ness of the loess, so long a time must have elapsed during its 

 deposition, that when the ground became elevated, a climate 

 must have existed not differing materiallv from that which 

 prevails to-day.* 



This tentative explanation of the forest growth, both in 



bottom 



and 



m both regions. From early days vhe origin of prairies has 

 been a source of speculation and many ingenious theories 

 have been brought forward. Of special interest are the 



* In connection with this see the interesting papers of Adams and 

 others, references to which may be found in Adams, C. C. The post- 

 glacial Dispersal of the North American Biota. Biol. Bull 9 : 53 1905 



