204 BOTANICAL GAZETTE [SEPTEMBER 
formations, is represented in great strength. In going north- 
ward, ascending the plains, this vegetation becomes gradually 
more dilute, finally leaving a pure grass plain. The most 
apparent index of transition is furnished by the mesquite, which 
persists long after every other Lower Sonoran shrub has disap- 
peared. Leaving a jungle of large mesquite below the escarp- 
ment, one comes upon a reduced growth upon the plains above, 
which becomes more and more dwarfed until at 3500 feet it is a 
mere clump of switches a foot tall, disappearing then absolutely. 
The same transition occurs in ascending the plains from the red 
beds province north of Red river, and where the Canadian cuts 
its cafion across the plains a Lower Sonoran flora has followed. 
The grass formation of the summit plains is a solid grass 
floor of the short buffalo grass type. These grasses, in harmony 
with the sudden transition from moist to arid conditions, are 
adjusted to rapid changes from the dormant condition to luxu- 
riant growth, and with the well-known characteristic that the 
growth already made when dry periods come on becomes pet- 
fectly cured as it stands, retaining all of the nutritious qualities 
which make these grasses economically valuable. All that 
remains alive during the dry periods are segments of the stem, 
which may be regarded as equivalent to buds or tubers oF 
rhizomes. These are especially rich in carbohydrates. They 
are not underground nor connected with the soil by living parts, 
but merely anchored by the dead fibrous roots of the old growth. 
The cured grass serves naturally as a protective covering for the 
dormant living parts, although these are not often killed when 
the dry covering is burned off. Indeed, it is this quality of 
being able to endure fires that gives to these grasses the great 
advantage they possess in their constant struggle against woody 
vegetation (fig. 23). 
The herbaceous species which accompany the grass vegeta- 
tion are chiefly annuals. The lignescent rooted perennials so 
characteristic of the Cretaceous plains and prairies are mostly 
wanting, which is in harmony with the open porous soil texture. 
The very abundant Yucca angustifolia is an exception to this 
