530 VEGETATION OF SOUTHWESTERN TEXAS. 
Grande, or most probably by the northern slope of the Mexican table-land southwest of that river ; 
and to the northwest by the mountains and plains of New Mexico. 
The geological character of this country is well known through the investigations of Prof. 
Roemer, to which the notes of Lindheimer and others have added some details. The whole of this 
country, with slight exception, is of cretaceous formation; the rocks are calcareous, with cherty mix- 
ture, and of horizontal stratification, rising in several plateaus or terraces one above the other, over 
the alluvial lands of the sea-coast; the plateaus often with steep declivities, and not rarely broken 
up into level topped hills with terraced slopes. The rivers and brooks are clear streams often with 
wide, shallow, gravelly beds, which, becoming dry, are exposed a great part of the year, and having 
on many places perpendicular rocky banks. Along the fertile margins of the larger streams the 
usual forest trees of the Southern States are found, not unfrequently also with the cypress (7axodium 
distichum), and on the slopes, fine cedar woods (Juniperus Virginianus) frequently close the view ; 
other pine trees are unknown in this district. On the thin soil of some parts of the country, 
scattering post-oak woods occur, but on the whole larger timber is scarce. 
The climate is mild, not too warm in summer, — in fact, it is said to be not as warm as that of 
the central portion of the Mississippi valley is from June to August; in winter occasionally ice is seen, 
but quickly disappears. Farther to the northwest the extremes are greater, and in the southern part 
of this region frosts are unknown. Spring opens with February, rains occur at that time and often 
as late as May and June; a season of general drought follows until the September rains again revive 
the vegetation. It is then that early flowering shrubs often once more begin to blossom, and many 
annuals again sprout up and bear flowers for the second time, assuming an almost ligneous stem 
and apparently perennial root, so that it sometimes becomes difficult to decide whether they 
really are annuals. Some plants, the representatives of which farther north blossom in spring, [225] 
put forth their flowers only in September or October; as Ovalis vespertilionis, the near relative 
of the common 0. violacea ; and Ulmus crassifolia, closely allied to U. alata. 
As we may expect, the Flora of Southwestern Texas partakes to some extent of the character 
of the vegetation of the adjoining districts. We find there the @nothere and Gaure, the Buffalo- 
grass, and many other plants from the northern plains. The now well-known Compass Plant of the 
prairies (Silphium laciniatum) is also found there! The western mountain region sends down to 
the caleareous plateaus a number of grasses, Yucca angustifolia, some Portulacacew, Nyctaginacee, 
and many others. 
From the table-lands of Mexico, and particularly its northern slopes, we find in the region we 
have now in view the Bolivariw, some Malpighiacee, some Zygophyllacee, besides the Mimosee 
and Cactacew, to be mentioned more fully hereafter; mostly genera or families not found farther 
north or east, but species not occurring farther south. 
Many herbaceous plants of different families are quite peculiar to that country. Of these I will 
mention only Rutosma Texanum, which with Peganum Mexicanum, A. Gray, mss., found south of 
the Rio Grande is, in America, the only representative of the Rue family. So are the congeners of 
the remarkable Hermannia Texana, common only in South Africa. 
But the striking character of the vegetation of that region consists in the diminution of trees 
into shrubs ; they do not disappear entirely as in the western plains and deserts, which extend into 
northwestern Texas; nor are they generally the same species which elsewhere grow up to be large 
1 It is perhaps not without interest to learn that Mr. of the noonday sun by presenting to it the edge of the leaf, 
Lindheimer has, with the com verified the so-called or both causes together, I leave swidatetinitied. We, at all 
polarity of the tata leaves of this plant. I have no doubt events, owe our thanks to Major Alvord for having, with a 
but that sunlight is the determining cause of this peculiarity, pertinacity which only a thorough conviction of the truth of 
as my friend Mr. I. A. Lapham of Milwaukee has suggested. the fact could have given him, forced the knowledge of a 
But whether it is the desire, if I may so express myself, of circumstance so interesting to physiological botany on the 
obtaining as much light as possible on both surfaces of the unwilling mind of naturalists. 
leaf, or perhaps rather a tendency to avoid the burning rays 
