360 VEsSTAL: A BLACK-SOIL PRAIRIE STATION 
tually pure growth of Glyceria nervata, which thus forms an inner 
zone. The large depression in the map has a deeper region occu- 
pied by a smartweed with large hairy leaves (Polygonum amphi- 
bium var. Hariwrightit) . 
Disturbed prairie—Ruderal and primitive prairie growths 
which come up in broken or otherwise modified prairie ground are 
often dominated by single plant species. The native prairie plants 
include the following: Lepachys pinnata, forming conspicuous pure 
growths, usually in rather dry soil; Asclepias verticillata, and 
Lespedeza capitata, locally abundant (rather infrequently in eastern 
Du Page County) in disturbed prairie along rights-of-way; Heli- 
anthus grosse-serratus, forming tall growths in rather moist soil; 
Cassia Chamaecrista, Erigeron annuus, and Ambrosia artemisu- 
folia, not very frequent; Erigeron canadensis, and Lactuca cana- 
densis, acting quite like successful introduced weeds; Aster multi- 
florus, persisting as a prairie relic even in city lots; and Convolvulus 
sepium, an insidious creeping dominant which replaces prairie 
plants under changed conditions. 
The prominent introduced plants are Agrostis alba and Trifolium 
repens in moist soil, especially with grazing; Phleum pratense and 
Trifolium pratense, hay plants common in rights-of-way, and in 
prairie which is occasionally cut for hay; Melilotus alba, a serious 
pest which has replaced extensive areas of prairie (once the prairie 
is broken, the sweet clover may assume complete dominance) ; Poa 
pratensis, very generally distributed, perhaps succeeding sweet 
clover after some years, in many stations; Daucus Carota and a 
number of other plants, infrequent. 
Development of the prairie—Mesophytic black-soil prairie may 
develop from either of two extreme types of vegetation, hydro- 
phytic or xerophytic. The developmental series beginning in 
shallow water or marshy situations has been discussed by Cowles 
(I. c., pp. 155-156). Gates has described succession from marsh 
associations to the Liatris spicata type of prairie (I. c., p- 335, Pl 
39). The development from grassland of pronounced xerophytic 
type has hardly been mentioned, with exception of that which 
takes place in prairie of dry sandy soil (sand prairie). The de- 
velopment of open xerophytic bunch-grass of sand prairie into 
less xerophytic types such as the Sporobolus heterolepis-Sorghas- 
