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, ete. 
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); Alisma 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 
