1906] SHANTZ—VEGETATION OF THE MESA 41 
beginning of this period. The grasses are dried, and although 
they are still a prominent part of the vegetation, they are not a living 
part. It is an exceedingly xerophytic time, and the plants which 
are found in this aspect appear throughout the vernal and aestival 
periods and are now only blooming and ripening their seeds. 
Artemisia frigida and Muhlenbergia gracillima continue to occupy 
a most important place. Senecio spartioides forms in places an 
extensive society; while the Gutierrezia Sarothrae society is even 
more noticeable than during the aestival period. Chrysothamnus 
graveolens, a large shrubby composite, forms a small society within 
this region, but farther east occupies larger areas; it is one of the 
most showy plants of this aspect. Aster polycephalus and Machae- 
ranthera cichoriacea form rather extensive communities in the more 
open parts of the formation. Chrysopsis villosa, Lacinaria punctata, 
Oreocarya thyrsiflora, Eriogonum Jamesii, Tetraneuris glabriuscula, 
Petalostemon oligophyllus, P. purpureus, Aristida longiseta, Grin- 
delia squarrosa, G. squarrosa nuda, Artemisia canadensis, and A. 
§naphalodes have continued from the preceding pericd. 
The end of this period is not well marked. The plants are dry 
and resistant, and although frost kills the plants which have a more 
liberal supply of water, some of these species may continue to bloom 
as late as December 10. During this late period Senecio oblanceo- 
latus, Argemone intermedia, Lesquerella montana, and a number 
of other species form rosettes which continue throughout the winter. 
Structure of the formation as illustrated by typical quadrats. 
Passing now from the aspects to the formation as a whole, the 
structure may be illustrated best by a number of permanent quad- 
rats. Those species which form mats cannot be well represented 
‘n numbers per square meter, and on this account the percentage 
of surface covered is given instead. The numbers which are also 
Siven for these species indicate single plants or seedlings. An esti- 
mate is also given of the total amount of surface covered by plant 
growth. 
The following quadrat is typical of the Bouteloua oligostachya 
“onsocies—the most typical portion of the Bouteloua formation. 
