BOTANICAL GAZETTE 



98 



remarkable for the general flowering of Poa pratensis. P. compressa 

 is associated, but occurs sparsely and principally upon the upper 

 slopes. The relative place of these two forms has been hard to deter- 

 mine, but it seems that P. compressa occurs as a forerunner of the 

 bluegrass. In its distribution P. pratensis is confined to depressions 

 and lower slopes, and is always indicative of the highest soil-water 

 content, the most favorable exposure, and richest humus. In these 

 situations it forms a dense sod, frequently exclusive, which is con- 

 stantly pushing up the slopes replacing the bunch-grasses, outlying 

 individuals frequently reaching the higher slopes. It is perhaps one 

 of the most mesophytic of prairie forms, and is almost invariably the 

 forerunner of the prairie shrubs, Symphoricarpus occidentalis and 

 Rhus glabra. It reaches its highest development on the northwest 

 exposure, and it is up this exposure that the flood-plain and ravine 

 forest of this region has made its greatest advance upon the prairie. 

 Poa is the first facies to bloom, but as noted above is of restricted 

 distribution. It is a perennial and propagation is rapid by the abun- 

 dant rootstocks, which leads to dense sod. 



With the bluegrass are successively added Oxalis stricla, Linum 

 rigidum, and Poly gala alba. Never very abundant and of low 

 stature, they add but little to the tone of the aspect, as they are over- 

 topped by the oncoming facies of later aspects. Oxalis appears on 

 lower slopes, possessing frequently a gregarious distribution. It may 

 be either annual or perennial. The few seeds are restrictedly dis- 

 persed by the dehiscing capsules. Linum is a decidedly xerophytic 

 annual, being a relict of earlier stages. It appears in the open asso- 

 ciation on the upper slopes and along prairie crests. It is never 

 common and its fugacious petals prevent all but entire inconspicu- 

 ousness. Its seeds are few and their mobility little. Polygala is 

 sparingly distributed over the lower slopes of prairie knolls, always 

 working up to higher positions with the increase of mesophytic 

 conditions, but it never remains in the Poa sod. It is perhaps one 

 of the best indices of progressive mesophytism among the prairie 

 species. It is a perennial from woody rootstocks. Seeds are borne 

 two in a capsule and migration is very slow. 



As the vernal floral aspect is drawing to a close, several minor 

 elements bloom, extending vernal floral activity over into the first 



