IOWA ACADEMY OF SCIENCES. 171 



from the Missouri fills up tiie nearly level plain forming the 

 lakes in which an abundant hyclrophytic vegetation occurs. 

 As these ancient lakes became gradually filled with organic 

 Qiatter, herbaceous plants similar to those of the Missouri 

 flood plain appeared. The continued increase of organic 

 matter made a soil more suitable for prairie plants of a 

 different character. Of the vernal plants we may note the 

 Banunculus septetitrionalls, Senecio aureus var. Balsamite, 

 the latter forming distinctive features of these meadows, 

 frequently producing masses of yellow flowers. The 

 Anenio)ie Fennsi/lra)ilea also forms solid masses. Thalidriim 

 jjurpurasceiis, Tleiichera villosa, Asclepias syriaca, Silphium 

 laciniatuni. These form great masses in the moister places. 

 Phlox pilosa grows abundantly in the prairie meadows and 

 is distinctively a prairie mesophytic plant, like Poa praten- 

 sis. HeliantJius grosse-se'rratus. Of the younger formation 

 distinctly hydrophytic we may mention 



Rumex vcticillatus . Glvceria fluitans. 



Phalaris arundinacea . Typha latifolia. 



The Boyer valley is marked by its prairie-like meadows 

 intersected by very small ravines. The deposition of 

 humus and considerable moisture in the soil prevents the 

 rapid desintegration of organic matter, hence uusuited for 

 the growth of trees and shrubs, but well adapted for species 

 of Care.r, E/j/ums robusfns, Phlox pilosa, Anemone Pennsyl- 

 vanica and Senecio aureus. 



Towards the approach of the bluff formation on either 

 side of the valley the drainage is more perfect, and tree 

 and shrub life begins. Unlike the younger flood plain of 

 :he Missouri a narrow zone of forest growth skirts the 

 Boyer. This forest area is not making much encroachment 

 upon the prairie flood plain. 



The soil along the stream is of much more recent for- 

 mation than the prairie flood plains. To the gradual 

 sloping banks there is added from year to year more black 

 alluvial deposit. At first such plants as Erar/rostis reptans, 

 Mimulus ringens, Pohjgonu)n acre. Of the later autumn 

 plants to appear here are Helianthiis grosse-serratus and 



