1917] VESTAL~—FOOTHILLS VEGETATION 373 
Ceanothus mollissimus and what appears to be Ceanothus subsericeus Rydb. 
occasionally occur with the spiny species, in the less xerophytic stations. 
Ceanothus velutinus is rare in the lower foothills, but is frequent at higher 
elevations and farther north. Herbs of the primitive grassland and mixed 
grassland commonly grow out from between the twigs of Ceanothus Fendleri, 
and to some extent are seen in the spaces between the mats. 
FOOTHILLS MIXED GRASSLAND ASSOCIATION 
CLEMENTS (1), ground-cover in the foothill thicket and pine formations, 
in part; RAMALEY (12), foothill sagebrush-grass formation; SCHNEIDER (21), 
half gravel slide formation, and grassland of north slopes; SHANTZ (22), 
outeloua formation, in part: its modified form at the mountain-front; VESTAL 
(24, p. 386), Bouteloua mixed consocies, as developed at the mountain-front; 
WATSON (25, pp. 209, 210), herbaceous ground-cover in the yellow pine asso- 
ciation, and mountain “meadows.” 
The mixed grassland association normally develops from primi- 
tive grassland, one of its important féatures being the establishment 
of the dominant Bouteloua oligostachya, the grama grass of the 
plains. It thus differs from the primitive grassland in that (1) 
the ground cover is less open, though still generally xerophytic; 
(2) the soil is usually more stable (in most situations a physical 
cause, rather than the effect, of the more permanent vegetation) ; 
(3) the soil is more finely broken up, and to it may be added con- 
siderable humus; and (4) a number of plains, prairie, and foothill 
species absent or rare in the primitive grassland are established. 
The assemblage is most heterogeneous, since the many plants include 
widely diverse ecological, geographic, and floristic types. Extreme xerophytes 
and relatively mesophytic plants, plants of widely varying growth-form and 
seasonal relations, of great difference in plasticity to environmental variation, 
in altitudinal and habitat range, may occur in the same small grassland area. 
This mixed vegetation is really very closely allied to the modified plains grass- 
land mentioned as the Bouteloua mixed consocies of the short-grass association 
(24, p. 386). This is found in the mixed mesa soils of the mountain-front zone 
eae oui the foothills. The conditions which would result in heterogeneity 
of th tation are probably similar in the lower foothills to those of 
= mountain-front; some of these are given in the article cited (24, pp. 381, 
82). 
. So many species occur regularly in the mixed grassland, and the variability 
in ‘flocistic composition in particular stations is so great, that a selected list 
ee habitats cannot be 
