52 BOTANICAL GAZETTE [JULY 
xerophytic situations; Pentstemon Jamesii, on clay knolls in the 
gullies of the south Mesa; Malvastrum coccineum, in semiruderal 
situations; Gaura coccinea and Thelesperma gracile, on open coarse 
gravel of north Mesa; Poa longipeduncula, on north slopes; Salvia 
lanceolata, in semiruderal situations; and Antennaria imbricata, 
on northwest and rarely northeast slopes. : 
Most of these communities are closed and contain very few if i 
any other species belonging to this aspect. Any of the species — 
which form societies will also be found to form communities; in 
fact some of the societies are merely associations of communities. 
General. a. 
The chief societies of the Mesa top are the Yucca glauca (fig. 6)| 
and the Senecio oblanceolatus societies. These become more or less a 
mixed in places on the north Mesa. In many places there is formed ; 
a mixed society which varies greatly and can only be regarded as 
a society made up by mixing the other societies and by the addition 
of widely distributed secondary species. The Mesa top contains — 
besides the two prominent societies mentioned above, a society” 
marked by Euphorbia robusta. These three societies alternate, | 
the last named occurring on the north part, the more open part of 
the formation, in gravelly loam, where the water content is from 
6-10 per cent. The Yucca glauca society occupies a somewhat more 
humid region—the crests and Mesa top, especially where the soil 
is a rocky, gravelly loam, with water content from 7-12 per cent. 4 
The Senecio oblanceolatus society reaches its maximum development 
in the gullies, where the loam has a water content of 10-15 per cent 
In addition to these well-marked societies, the following plants 
occur in varying numbers over practically the whole Mesa top 
Lesquerella montana, Pentstemon angustifolius, Aragallus Lamberti 
Astragalus crassicar pus, Oreocarya thyrsiflora, Erigeron pumilus, Eq 
canus, Astragalus Shortianus, and Mertensia linearis. Any oF all 
of these species may occur in the societies noted above. 
The gravel crests with a water content of from 4. 5-9 per cent 
are the most xerophytic situations in the formation; and here are 
found several societies that alternate. Most of the crests of the 
north and east sides of the Mesa are occupied by the Penistemonm 
