Every observing traveller must have noticed how closely the peculiarities of the scenery of a country depend upon its vegetable productions. Not only is this true of trees and the larger forms of plants, but even the humbler, though apparently insignificant in themselves, have their share in producing the general effect. Hence the subject of the geographical distribution of plants is one which may command the attention of every intelligent mind, and this department of botany has for many an interest which the higher details of the science do not possess. Perhaps no region of equal extent presents more marked illustrations of the relation of the vegetation of a country to its topography and geology than those afforded by that lying along the Mexican boundary line.

The usually difficult task of constructing a phytological map might here be performed with comparative ease, as the observer, little perplexed by a great variety or gradual blending of forms, involuntarily associates particular localities with the predominating and characteristic vegetable productions. Thus one who has ever traversed the desert table lands of the Upper Rio Grande will not fail to unite in his recollection of these tracts the dull foliage of the Creosote bush, the long thorny wands of the Fouquieria, the palm-like Yucca, and the crimson-flowered and spine-armed Cereus. Still less can any one, who has seen the giant cactus of the Gila in its perfection, ever forget the wild and singular features of the country in which it grows. The distinctness with which the botanical districts are defined gives an unpleasant sameness to the scenery of this country. The extensive plains exhibit a monotonous succession of the same forms, and each mountain slope and ravine presents us a collection of plants quite like those we have so often seen in other and similar localities. Indeed, the botanist in these regions, knowing what to expect in each different situation, soon loses his zeal, and becomes intent upon little else than overcoming space.

We propose to give a rapid sketch of the features presented by the vegetation of the country, especially noticing those plants which predominate in, and give character to, the several districts into which we have divided it.

THE LOWER MARITIME BELT.

The flora of this strip of country is too well known to require an account here, had we the necessary data for describing its characteristic botanical productions. Its position being intermediate between the "tierra caliente" of Mexico and the gulf coast of Louisiana, gives to its vegetation a mixed character, partaking of that of the two extremes. Thus, while upon the

lower portion of the Rio Grande, we have the sub-tropical character of its vegetation indicated by the presence of the arborescent palms of the "tierra caliente," we have at the same time the Texas live-oak, the type of a more northern flora. This region has been more thoroughly explored by A. F. Schott, esq., whose observations upon its botany and other natural features will be found in a separate memoir.

UPPER MARITIME BELT.

To the lower belt, which has an uniform alluvial soil, and is more or less influenced by its vicinity to salt water, succeeds rolling prairies, underlaid by cretaceous rocks, which now for the first time appear, though more or less hidden from view by a thick layer of erratic and alluvial deposits. This division includes, as we have already noticed, the most habitable parts of Texas, through which run the numerous rivers which empty into the Gulf of Mexico. Here we find rich cotton lands, and an abundance of trees, including species of oak, hickory, ash, elm, walnut, cypress, &c., with an exceedingly rich undergrowth of vines and shrubbery. The open prairies are densely covered with luxuriant grasses, and have a rich and varied flora, which has been well explored by the early labors of Drummond, and later by those of Linctheimer and Wright. Upon the southern portion of the Rio Grande, where there is a higher temperature, united with greater aridity of the soil, a vegetation of quite a different character appears; we have here the dense growth of "chapparral," which is so peculiar to this region. The plants which make up the thick mass of shrubbery known by this name are different species of mimosa and acacia, with the well known mezquit and other forms, most of which are armed with hooked thorns, and make up a jungle which is almost impenetrable.

The botany of this region is too well known, from various published accounts, to require further details in this place.

VEGETATION OF THE CRETACEOUS FORMATION.

As the geological formation becomes more exclusively cretaceous the vegetation assumes a peculiar character, and is made up of species which are rarely found elsewhere. The shrubbery of this region presents a continuous succession of the same forms, among which are Berberis trifoliolata, Rhus microphylla, Porliera angustifolia, Diospyros Texana, Roeberlinia spinosa, Adolphia infesta, Microrhamnus ericoides, and Acanthoceltis, a new genus of Engelmann, allied to Celtis. Along the margins of the usually dry water courses the dwarf walnut (Juglans rupestris) and Fallugia paradoxa are constantly found.

The perennial herbaceous plants of this district are numerous. The rocky ledges produce in their crevices various species of Laphamia and the scarlet flowered Pentstemon Grahamii, Several examples of the large tropical family of Malpighiacea are found here, among them Galphimia linifolia, Aspicarpa hyssopifolia, and Janusia gracilis. Cacti are numerous, and include among the forms here presented opuntias, mammillarias, and numerous species of cereus. The curious Lycopodium dendrolobium grows upon the perpendicular faces of the limestone rocks. This plant commonly called "rock rose," is remarkable for the hygrometric qualities of its fronds, which are usually rolled up so that the plant forms a dry ball, which expands under the influence of moisture, and then appears fresh and vigorous. Several new species of Cheilanthes, Pteris, Notoclana, and other genera of ferns are found here.

Upon the rocky ledges a small species of agave grows in abundance. The low leaves, which are pointed with sharp spines, are very troublesome to the foot traveller; they are, however, of some use to the Mexicans, who employ the strong fibres they contain in making coarse ropes. The plant is known to the people of the country as "Lechaguia." The table lands and valleys are usually covered with an abundant growth of "grama grass," and among it are frequent clumps of Dasylirium, or "bear's grass.

We have here also several interesting species of Nyctaginaceae, belonging to the new genera, Acleisanthes, Pentacrophis, and Selenocarpus, lately proposed by Dr. Gray.

Among the annual plants of this district are several species of Mentzelia, Perezia, Pectis, Hymenatherum, &c., also the pretty Eucnide lobata, which usually grows in almost inaccessible situations upon the perpendicular faces of the limestone ledges.

There are so many species that seem to be equally abundant that it is difficult to designate any as being characteristic of the cretaceous district.

VEGETATION OF THE HIGHER BASIN PLAINS AND THE ADJOINING MOUNTAIN RIDGES.

The higher alluvial tracts, forming the basin plains before described, produce a number of northern forms of plants, such as species of Enothera, Gaura, Riddellia, Zinnia, and Polygala. We also find here the curious Peganum Mexicanum and Peteria scoparia, the latter a pretty plant of the family Leguminosa. The depressions in this alluvial region are covered with a coarse grass, which presents an uniform dead brown color throughout the greater part of the year. In the deep recesses and shaded valleys, the vegetation has a freshness unknown to that of the plains. In these localities we encounter the upland live oak (Quercus Emoryi, and the nut pine, (Pinus edulis,) and growing beneath these, Vitis incisa, Clematis Pitcheri, Ungnadia speciosa, &c. The constant presence of water in the larger valleys is marked by the growth of cotton wood and willows.

VEGETATION OF THE EL PASO BASIN AND THE UPPER RIO GRANDE VALLEY.

The vegetation of the immediate valley of the Rio Grande, and that of the country immediately adjoining it upon either side, are strikingly different, and whoever passes from the valley, and crosses the line of mountains which bounds it to the regions beyond, cannot but remark the difference in the landscape, due to the presence of new plants. Upon the table lands which spread out beyond the mountain barrier, the eye falls upon a great variety of plants, none of which are seen in the more fertile valley. Among these are Fouquieria splendens, Larrea mexicana, Flourensia cernua, Rhus microphylla, Condalia obovata, Koeberlinia spinosa, and species of Krameria, Ephedra, and Yucca. There the Cacti flourish in a congenial soil, and we find representatives of the genera Opuntia, Echinocactus, Mammillaria and Cereus.

Among the numerous herbaceous and suffruticose plants of these localities we may mention Cevallia sinuata, Greggid camporum, Eriogonum Abertii, and several species of Dalea; plants of the family Compositae, are especially abundant, and include among others Baileya multiradiata, Bahia absinthifolia, Porophyllum scoparium, Psathyrotes scaposa, Hymenatherum acerosum, Townsendia strigosa, Calycoseris Wrightii, Stephanomeria minor and Rafinesquia Neo-mexicana. The natural order of Nyctaginaceae is represented by Selenocarpus chenopodioides, Boerhaavia Wrightii, and others.

The principal grasses of this region consist of the kinds known as "bunch grass," and belong to the genera Chondrosium and Bouteloua. The margin of the table land, where it

borders upon the valley is broken by deep ravines, and we find here upon the sandy bluffs a growth of chaparral, made up principally of mezquit and the equally thorny acacias. In the dry pebbly water courses, the willow-leaved Chilopsis linearis frequently occurs. Many of the already mentioned plants of the table land are found here, besides others, which are peculiar to these localities, such as Dithyræe wislizenii, Abronia mellifera, Gilia longiflora, Lepidium alyssoides, Gaillardia pinnatifida, Palafoxia Hookeriana and Tetraclea Wrightii, a singular genus of Verbenaceae, recently described by Dr. Gray. In the valley of the Rio Grande we frequently find a heavy growth of cotton wood and willows. The "screw bean," Strombocarpa pubescens, often occupies large tracts, accompanied by a dense undergrowth of Baccharis salicina. The low saline places produce an abundance of Obione canescens, while on the higher ground, Tessaria borealis is a common plant.

A number of coarse composite plants are found in the valley, such as Texmenia encelioides, Coreopsis cardaminefolia and Aster spinosus, the latter often forming dense matted masses of considerable extent.

The rocky crevices of the adjoining mountains furnish some of the most interesting plants of this region. We here find Fendleria rupicola, Mortonia crassifolia, Glossopetalon spinescens, Agenia parvifolia, Bouvardia hirtella, Tecoma stans, Texmenia brevifolia, &c.

The higher mountains of the Organ range have a vegetation possessing a somewhat alpine character, and bear a scattering growth of pines and oaks, beneath which flourish a number of shrubby and herbaceous plants, quite similar to those found upon the more lofty ranges further to the west.

VEGETATION OF THE SIERRA MADRE.

As we approach the base of the Sierra Madre, passing over the extensive basin-plain already described, a gradual increase in the elevation is marked by a disappearance of those plants, which were common lower down, and the presence of others wearing a more alpine aspect. The surface is less bare than upon the plain below, and is covered with a closely matted grass, which gives a singularly rounded outline to the undulating land around the base of the mountain. Along the lower rocky swells grows the beautiful Carphochete Bigelovii, with Anemone Caroliniana, Streptanthus linearifolius, Pentstemon Torreyi and Fendleri are among the characteristic plants of these localities. There are but few shrubby plants, several species of Ephedra being among the most conspicuous. The streams are bordered by Fraxinus velutinus and Juglans rupestris, (the large variety,) and in moist places an old Californian acquaintance, Anemiopsis Californica makes its appearance.

Upon the mountains oaks and pines are found, mostly Quercus Emoryi and Pinus edulis, though in certain localities there is larger timber, consisting of Pinus Chihuahuana and Abies Douglasii. The smaller woody growth here includes several species belonging to the Californian mountain flora, such as Cercocarpus parvifolius and Arctostaphylos tomentosus.

Upon the summit of Ben More, further to the north, Dr. Bigelow discovered many plants indicative of an alpine flora, among these were Frasera speciosa, Rubus Neo-mexicanus, Actinella Richardsonii, and, perhaps, most remarkable of all, a small fern, Asplenium septentrionale, an European species not before detected on this continent.

The abrupt descent through Guadaloupe pass, upon the western slope of the ridge, presents a profusion of evergreen shrubs and flowering plants, including most of those which occur on

the summit ridge. Live oaks grow upon the rocky sides of the ravines, and with them is frequently associated a species of cedar. Among the more interesting of the herbaceous plants found here, are Dryopetalon runcinatum, Vesicaria, and Colomice. The narrow valley of the stream which runs through the bottom of the Pass is wooded with ash, cotton wood, and Mexican

sycamore.

VEGETATION OF THE UPPER VALLEYS OF SAN BERNADINO, SAN PEDRO, AND SANTA CRUZ WITH THAT OF
THE ADJOINING MOUNTAIN RANGES.

The country embraced in the above limits, includes that portion of Northern Sonora, which divides the waters which flow north towards the Gila river from those whose course is south, towards the Gulf of California. It is, as we have before stated, diversified with high wooded mountains and upland plains, well watered valleys and dry and barren tracts. The arborescent growth is not essentially different from that we have noticed in speaking of the other divisions of country. Live oaks, the nut pine, cedar, ash, walnut and cotton wood are produced either upon the mountains or in the upland valleys. Its plains are covered with an uniform growth of upland grama grass, or in the more arid localities by mezquit and its thorny associates. This region furnishes a number of singular and highly interesting genera and species, most of which are described by Dr. Gray, in the second part of "Plantae Wrightianæ,' in the Smithsonian Contributions. A reference to this work will give a better idea of the character and distribution of the flora of this district than our limits will allow us. As it occupies a station between several botanical divisions, so its flora partakes of that of those regions. The following list of some of the plants found here will be seen to embrace species belonging to California, Texas, Mexico, and New Mexico, viz: Eschscholtzia douglasii, Zauschneria Californica, Eulobus Californicus, Bowlesia tenera, Anemone Caroliniana, Draba caroliniana, Corydalis aurea, Androsace occidentalis, Rutosma texana, Erodium texanum, Layia Neo-mexicana, Cowania Mexicana.

VEGETATION OF THE LOWER SANTA CRUZ VALLEY, THE DESERT OF TUCSON, AND THE CENTRAL VALLEY
OF THE GILA.

The region defined as above constitutes a very distinct botanical district, many of its peculiar plants not being found elsewhere. As we have mentioned in a previous sketch, the valley of the Santa Cruz, as it leaves the mountains in its northward course towards the Gila, gradually looses its fertile character, and finally terminates in the desert plain which forms the table land. of the Gila. The vegetation of this tract comprises many of the forms which are found on all the barren plains of the country. Here, as in similar situations elsewhere, the mezquit and the creosote bush are conspicuous. Cacti are abundant here, and of various types. There are the low arborescent Opuntias, generally bearing proliferous fruit, as well as several ellipticalstemmed species. The enormous Echinocactus wislizenii and Caespitose mammillarius and Cerei are common, while, either standing solitary or collected in groups, the lofty Cereus giganteus towers above all. A species of misletoe is common upon the mezquit trees of this region.

The sheltered crevices of the neighboring mountains produce a species of agave, and in these localities we find Franseria deltoidea, Encelia farinosa, and Perityle nuda.

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