284 BOTANICAL GAZETTE [OCTOBER 
indicate about the ultimate point of extreme exposure to intense 
light, burning midday and chilling midnight temperatures, and 
moistureless air and soil. 
There is still another and very striking species associated 
with the cactus-yucca-agave formation, especially on the most 
arid stony mesas and slopes where cacti predominate. This is 
Fouquiera splendens. Its slender, thorny, wand-like stems arise 
in small groups from the surface of the ground to the height of 
five to eight feet, bearing leaves only in a tuft at the apex. It 
is as if the most arid spots had been located and marked out by 
this plant (jig. 20). 
ROCK VEGETATION OF CRYPTOGAMOUS PLANTS. 
The formation of succulent vegetation might be designated, 
on the whole, rock vegetation, occurring as it does either upon 
stony talus or on cafion bluffs. Certainly many succulents not 
mentioned in the discussion are characteristic rock plants, but 
the attempt here is to select types of formation which in a 
peculiar or conspicuous way constitute the characteristic vegeta- 
tion. The xerophytic cryptogamous plants constitute such a 
vegetation. They are a rock vegetation either because they 
occur upon a bare rock substratum or because they inhabit clefts 
or fissures in the rocks, or dry rock ledges. The former play 
an important role in disintegrating solid rock, as the granitic 
lichens, or in promoting the accumulation of a matrix of organic 
material upon barren surfaces, as the Cyanophyceae on bare 
chalk or compact limestone débris. Throughout the central 
Texas Cretaceous areas the occurrence of these low plants is 
very noteworthy. They consist of a species of Nostoc which 
occurs in curled, crumpled masses after rains or foggy weather, 
literally covering the ground; a mat-forming Scytonema ; and 
several matted, leathery lichens, 
The so-called rock ferns are ecologically the most note- 
worthy elements of the vegetation of rock fissures and ledges. 
The several genera and numerous species of ferns constituting 
this xerophytic group are associated with the Sonoran climatic. 
