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Transactions Texas Academy of Science. 
dant areas for lechuguilla and the flats covered with fine sediment 
give Covillaea opportunity for its conspicnons occurrence. 
With regard to the formation as a whole; it strikes one as being 
notably a nonfoliaceous vegetation. The soil is not shaded and 
shade loving plants are wanting. This condition is of course due to 
the very large extent to which the foliage functions of leaves are 
subordinated to other purposes or to their great reduction in size or 
to their entire absence. Another striking characteristic of the for- 
mation is the fact that it almost never hides the ground upon which 
it stands — a circumstance due partly to the lack of foliage just 
mentioned, but especially to the actually unoccupied space. In 
this respect, the formation presents two phases, first that due to the 
presence of perennial species whose aspect varies only with the 
presence or absence of young vegetation or flower shoots, and 
second that due to the addition of the short lived annual species or 
herbaceous perennials whose presence is dependent upon copious 
rainfall. These appear on the whole more regularly in the spring, 
although the rainfall upon which thejr are dependent may come in 
September or October as in the season of 1904. The coming of 
the annuals tends to close up the formation by claiming unoccupied 
space between the perennial species and together with the young 
growths upon these latter approximate a closed formation after 
an unusual amount of well distributed rainfall. This may be called 
the vernal aspect of the vegetation even though its coming be de- 
layed until June or late September. Even at such times of rel- 
atively luxuriant growth, the vegetation is not sufficient to hide the 
white background of disintegrating limestone (or of other geolog- 
ical formations where these occur, e. g., in the G-reat Bend) while 
in the drought of midsummer and in winter the vegetation cover 
seems meager indeed. In any event, the arrival of the favorable 
temperature conditions of February and March results in the for- 
mation especially of flower shoots on the part of some of those 
species which have abundant storage tissues or those whose roots 
penetrate deeply enough do find a constant, if meager supply of 
moisture. Thus, Yucca treculeana, Hesperaloe parviflora, Cereus 
species, Microrhamnus ericoides, most Mimosae, Dalea frutescens, 
Covillaea and others were in blossom in early March of 1903 and 
1904 although it was then too dry for a general bursting forth of 
the vegetation: Other species require a higher temperature to 
induce the formation of flower shoots — e. g., most cacti, lechuguilla 
and sotol itself — and so do not blossom until June or later. 
Certain of the phenomena of association of species in the vegeta- 
