BIG SPRING PRAIRIE. 65 



No one species can be considered characteristic or 

 even common. Here and there a stray Erigeron, Po- 

 tentilla, Aster, Ambrosia, Capsella, Ranunculus, etc. 

 may occur. This is another striking illustration of the 

 results of location, environment, and direction of the 

 prevailing winds; and also of the fact that most seeds 

 require a bare soil for germination. A few smaller iso- 

 lated areas of this class occur with characters interme- 

 diate between the above form and the area just north of 

 Carey as figured on map II. 



THE DITCH SOCIETIES 



These would be divided into those plants growing 

 directly in the ditches, and those flourishing on the 

 banks. The principal plants growing directly in the 

 ditches of this prairie are: Lemna minor (Lesser 

 Duckweed); Bidens laevis (Smooth Bur-Marigold); 

 Coreopsis tinctoria (Garden Tickseed); Alisraa plan- 

 tago-aquatica (Water Plantain); Bidens frondosa (Beg- 

 gar-ticks); Isnardia palustris (Marsh Purslane); Lud- 

 wigia polycarpa (Many-fruited Ludwigia); Eupatorium 

 perfoliatum (Bone-set); Eupatorium maculatum (Spot- 

 ted Joe Pie Weed); Verbena hastata (Blue Vervain); 

 Xanthium strumarium (Cockle-bur); Homalocenchrus 

 oryzoides (Rice Cut-grass); Typha latifolia (Cat-tail); 

 Carex (numerous species) (Sedges); Bidens connata 

 (Swamp Beggar-ticks); Helenium autumnale (Sneeze- 

 weed) and Ambrosia trifida (Horse-weed). Along 

 the banks there flourish chiefly grasses, willows, 

 young elms, young cottonwoods, and a variable num- 

 ber of other species depending upon the nature of 

 the plant societies in the immediate vicinity. 



6. Tree Introduction upon Prairie, the Or- 

 der AND Cause. 



A number of theories have been advanced to ac- 

 count for the absence of trees upon prairies, but few 

 to account for tree encroachment upon same. Most 



