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Transactions Texas Academy of Science. 
ons as the gracefully spreading grass like leaves of sotol nor does 
the lechuguilla growth attain as great height as the sotol, averag- 
ing perhaps less than one foot, though in the Great Bend region — 
slope of the Chisos Mountains — the growth is taller, which is a 
matter of consequence in , connection with the yield of fibre. See 
plate III and pi. IV, fig. 1. 
Covillaea Association: This is the Mexican greasewood vegeta- 
tion which is a mark of the Lower Sonoran desert almost through- 
out. Its characteristic occurrence is upon fine soil covered flats 
where it forms a symmetrical open growth two and one-ha] f to 
three feet tall not unlike a young orchard plantation. Covillaea 
forms a characteristic feature of the bolson desert vegetation in 
West Texas — regions not comprised in the sotol formation. 
Mixed Association of Chaparral Yucca , Cacti ■, etc. : The sotol 
formation seems to be tending in the direction of becoming a more 
general mixture of associated species. The openness of the for- 
mation in general seems always to invite invasion of most species 
not already on the ground and the chaparral of far Western Texas 
seems to be almost as aggressively spreading as in the Bio Grande 
Plain and Central Texas. This mixed association is at present 
confined more to the depressions and the level divides where mois- 
ture conditions are better than on the sharper slopes and ridges. 
The drainage courses which in their lower stretches cut deep can- 
yons, are, on the upland, broad shallow depressions or valleys. In 
these depressions the mixed association has attained a characteristic 
development. Along the railway from Langtry to Sanderson this 
is especialty illustrated. Yucca treculeana becomes an emphatic 
feature in the landscape here. 
In a minor way Xolina, Fouquiera, Selaginella lepidophylla, 
Flourensia, Mimoseae, Leucophyllum and others become in turn 
the dominant species, and on the eastern border of the sotol country 
— e. g.'. about DelRio — haujillo and Parkinsonia texana give the 
vegetation aspect in places where one .would, expect the sotol and 
lechuguilla. See pi. VII, IX, X. 
The Occurrence of Cacti in the Vegetation : The part played by 
cacti in the sotol formation in Texas is relatively inconspicuous. 
It is well known, however, how completely they dominate the for- 
mation in the Mexican Plateau. In the region between the lower 
Devil'S Biver and Pecos Canyons there are probably fifty species 
of cacti. At no point do these constitute a dominant element in 
the formation (except for small areas of a few square yards) but 
they would be designated as of frequent or abundant occurrence. 
