532 GEOGRAPHICAL DISTRIBUTION OF THE FLORA OF NORTH AMERICA. 
whole continent. The northern forms that inhabit the Rio Grande country (and also those of 
New Mexico) show some peculiarities which distinguish them in some measure from those farther 
south. 
Several species of Opuntia, a genus which is more widely distributed over the continent than 
any other of the family, are found there; most of them of the usual well-known form, with flat 
joints; some prostrate on the ground; others erect, and often forming stems of six inches’ diameter 
(Opuntia Lindheimeri). Another singular tribe of Opuntia, with cylindrical joints, is there repre- 
sented by a single slender shrubby species (Opuntia frutescens), while to the west and south the 
splendid tree-like Opuntia arborescens borders along the confines of this region. Procumbent Opuntia, 
with short club-shaped joints, occurring with the last-named species, can hardly be said to belong 
to our district. 
A number of Mamillaria, several of them with large and beautiful flowers, are frequent there. 
A large cake-shaped Echinocactus (F. Texensis) is common all over that region, and a peculiar 
tribe of that genus, the most common of which is Z. setispinus, characterized by Heshy berries [228] 
(the fruits of the Hchinocactus are generally dry), covered with very thin membranaceous 
scales, appear to be characteristic of the Flora of the Rio Grande valley. But the most beautiful 
Cactacee of that country are the low, as it were stunted, almost globose species of Cereus (comprised 
under the subgenus Zchinocereus), with delicate flowers, often larger than the plant itself. These 
forms extend into New Mexico, and into the northern provinces of Mexico, but appear to be un- 
known farther south, where the species of this family are most fully and most numerously developed. 
No other genera of this peculiarly American family have been discovered in Texas. 
The southwestern part of Texas will soon become better known to us, through the continued 
exertions of Mr. Lindheimer, and through the praiseworthy exertions of Mr. Chas. Wright, who is 
now for the second time on his way from San Antonio to El Paso, and of Dr. Bigelow, of your State, 
attached to the Boundary Survey, who has excellent facilities for exploring the region along the 
Rio Grande River. 
IJ. GEOGRAPHICAL DISTRIBUTION OF THE NORTH AMERICAN FLORA.* 
FroM THE TRANSACTIONS OF THE ACADEMY OF SCIENCE oF St. Louts, Vou. III. 1877. 
Tr is well known that the broad belt of timberless land which stretches from the Hud- [eclxx] 
son Bay regions across the valleys of the Upper Mississippi, Missouri, and the western con- 
fluents of these rivers through Texas and into Mexico, separates the floras of the eastern wooded, 
from those of the western mountainous, regions of North America. This belt has a flora of its own, 
which, singularly enough, is little influenced by latitude, at least in its prevailing features, many of 
the characteristic plants stretching from Manitoba down to the Rio Grande. 
The Atlantic and Pacific floras are so completely separated by this belt that scarcely any 
species of tree of one side is found on the other. Our Red Cedar constitutes one of the very few 
exceptions. 
The Eastern Flora is generally divided into the strictly Atlantic Flora and that of the Mis- 
sissippi valley. The former includes that of the Alleghany Mountains down to their western de- 
clivities, and to the headwaters and upper valleys of the western streams. What we may call 
‘eG. y. 43. e 1 the range and migration of plants Muhlenbergii, which, however, also extends eastward, though 
and animals in the same number of the Transactions, Journal ina limited degree. The same may be said of Vitis Labrusca, 
of Proceedings, pp. eexxvii and cexxxi-cexxxii. — Eps. which is properly eastern, but is found also on the headwaters 
1 Among the interesting facts connected with this floral of streams flowing westward ; farther west it is represen 
limit is that the true Quercus Prinus is not found west of this by V. estivalis, common to both floras, some western forms 
‘line of demarcation, while westward it is represented by Q. of which have often been mistaken for Labrusea. 
