1906] SHANTZ—VEGETATION OF THE MESA 29 
lobata (Torr.) Raf., Leucolene ericoides (Torr.) Greene, Meriolix serrulata 
(Nutt.) Walp., Lithospermum linearifolium Goldie, Anogra albicaulis (Pursh) 
Britton, Poa longipeduncula Scribn., Salvia lanceolata Willd., Gaura parvi- 
flora Dougl., Hedeoma nana (Torr.) Greene, Antennaria imbricata A. Nels., 
Evolvulus pilosus Nutt., Thelesperma intermedium Rydb., Sitanion elymoides 
(Torr.) Greene, Oreocarya suffruticosa (Torr.) Greene. 
The vernal period, with which the prevernal is sometimes more 
or less blended, is ushered in by the spring rains and usually extends 
from about the first of May to the middle of July. The water content 
is higher at this period than any other, as is also the relative humid- 
ity. Extreme temperatures are not recorded, and the conditions for 
growth are more favorable than at any other time during the year. 
This aspect is marked by the appearance of a great number of 
seedlings and by many showy flowering plants. The floral display 
is almost entirely of perennial plants. The earliest species generally 
appear on the north slopes or north crests, a position protected 
from the high temperature and excessive loss of water during the 
winter period. 
Socteties. 
Senecio oblanceolatus society.—This society is by far the most 
important of the vernal period. It reaches its maximum develop- 
ment in the large gullies on the south side of the Mesa, but the species 
is distributed over practically the whole area. 
Yucca glauca society (fig. 7).—This species is one of the mcst con- 
spicuous plants of the Mesa region and in many places becomes 
dominant for this aspect. Since the plant is perennial, it is at all 
times one of the most characteristic of this part of the formation. 
Species of secondary importance in this society are Senecio 
oblanceolatus, Euphorbia robusta, Lesquerella montana, Echinocereus 
viridiflorus, and Mertensia linearis. This society is found on the 
gravelly soil, often on crests and slopes where the water content is 
especially low. 
Pentstemon angustijolius society——On crests in the south part 
of the Mesa this society reaches its best development. Here the 
species dominates areas of many square meters, almost to ‘the exclu- 
sion of any other species. The-chief secondary species of this society 
are Lesquerella montana and Echinocereus viridiflorus. 
