i9o8] HARVEY— PRAIRIE-GRASS FORMATION 279 



the chresard at a minimum are thus the ecological conditions which 

 strongly mark the scroti nal floral aspect. 



Species of the serotinal floral aspect 



Facies.— Bouteloua oligostachya,t B. hirsuta,t B. curtipendula,t Bulbilis 

 dactyloides.*t 



Primary species. — Kuhnistera purpurea,! Verbena stricta,*t Ratibida 

 columnaris,*t Kuhnistera candida,t Symphoricarpus occidentalis,*t Amorpha 

 canescens.*t 



Secondary species. — Agropyron occidentale, Carduus undulatus,* Euphor- 

 bia marginata,* Hymenopappus filifoHus, Calamovilfa longifolia, Polygala alba.* 



Tertiary species. — Lygodesmia juncea, Lacinaria squarrosa, Brauneria 

 pallida,* Meriolix serrulata,* Eriocarpum spinulosum, Erigeron ramosus,* 

 Potentilla Hippiana.* 



Ruderal species. — Cassia chamaecrista, Onagra biennis, Amaranthus 

 graecizans, Melilotus alba,* Chenopodium album, Lactuca canadensis, Apocynum 

 cannabinum. 



*From previous aspect. fForming associations. 



Agropyron occidentale is the first of the serotinal grasses to bloom. 

 It is a xerophytic bunch-grass and occupies prairie crests, where it 

 occurs copiously, rarely even of facial rank. It is one of the pioneers 

 of the bunch-grass stage and is associated with the Andropogons, 

 passing with these forms as they give place to the Boutelouas and 

 being entirely absent in the older and more mesophytic prairie. In 

 transitional stages from the bunch-grass open association to the less 

 xerophytic closed sod association, Agropyron remains not infre- 

 quently in subcopious abundance as a relict of the earHer condition. 

 The rootstock is here an efficient mode of propagation. 



The three grama grasses, Bouteloua hirsuta, B. oligostachya, and 

 B. curtipendula, which head out during the early days of July, enter 

 upon anthesis almost simultaneously with the beginning of the 

 second week of July, as does also the buffalo grass, Bulbilis dacty- 

 loides. The Boutelouas are pioneer sod-formers, following only the 

 buffalo grass, which as the pioneer sod-former encroaches upon the 

 bunch-grasses, replacing them and preparing the way for the Boute- 

 louas which invariably follow closely. To the Vest, where the rainfall 

 is much less, Bulbilis is the prominent sod-former and is the fodder 

 grass of the great cattle ranges west of the Missouri. In our region 

 it occurs along the xerophytic exposures of the bluff line, and as a 



