The delicate and feathery foliage of the Mimosæ, Acaciæ, Schrankiæ, and Desmanthus, may be regarded as peculiarly characteristic of tropical vegetation; although some representatives of this form may also be found without the tropics. In the Old Continent of the northern hemisphere, and indeed in Asia, I can instance only one low shrub, described by Marshal von Bieberstein as Acacia Stephaniana, but which, according to Kunth’s more recent investigations, is a species of the genus Prosopis. This social plant covers the arid plains of the province of Schirvan on the Kur (Cyrus), near New Schamach, as far as the ancient Araxes. Olivier found it also in the neighbourhood of Bagdad. It is the Acacia foliis bipinnatis mentioned by Buxbaum, and which extends towards the north as far as 42° lat.[[OK]] In Africa the Acacia gummifera (Willd.), extends to Mogador, and therefore as far as 32° north lat.
In the New Continent, Acacia glandulosa (Michaux), and A. brachyloba (Willd.), adorn the banks of the Mississippi and Tennessee, and the Savannahs of the Illinois. The Schrankia uncinata was found by Michaux to penetrate from Florida northwards to Virginia (therefore as far as 37° north lat.). Gleditschia triacanthos is met with, according to Barton, to the east of the Alleghany mountains, as far as 38° north lat., and west of the same range even to 41° north lat. The extreme northern limit of Gleditschia monosperma is two degrees further southward. Such are the boundaries of the Mimosa form in the northern hemisphere, while in the southern hemisphere, beyond the tropic of Capricorn, simple-leaved Acaciæ are found as far as Van Dieman’s Land; the Acacia cavenia described by Claude Gay being even found in Chili between 30° and 37° south lat.[[OL]] Chili has no true Mimosa, but three species of Acacia; and even in the north of Chili the Acacia cavenia grows only to a height of 12 or 13 feet, whilst in the south, as it approaches the sea-coast, it scarcely rises a foot above the ground. The most sensitive of the Mimosas which we saw in the northern portion of South America, are (next to the Mimosa pudica,) M. dormiens, M. somnians, and M. somniculosa. The irritability of the African sensitive plant was already noticed by Theophrastus (iv. 3), and by Pliny (xiii. 10); but I find the first description of the South American sensitive plants (Dormideras) in Herrera (Decad. ii. lib. iii. cap. 4). The plant first attracted the attention of the Spaniards, in 1518, in the Savannahs on the isthmus round Nombre de Dios (“parece como cosa sensible”), and it was pretended that the leaves (“de echura de una pluma de pajaros,”) only contracted together when they were touched with the finger, and not when brought in contact with a piece of wood. In the small swamps which surround the town of Mompox on the Magdalena River, we discovered a very beautiful aquatic Mimosa (Desmanthus lacustris), a representation of which is given in our “Plantes équinoxiales” (t. i. p. 55, pl. 16). In the chain of the Andes of Caxamarca we found two Alpine Mimosas (Mimosa montana and Acacia revoluta) growing at elevations of from 9000 to nearly 9600 feet above the level of the sea.
As yet no true Mimosa, (in the meaning of the word as established by Willdenow,) nor even any Inga, has been found in the temperate zone. Amongst all the Acacias the Oriental Acacia Julibrissin, which Forskäl has confounded with Mimosa arborea, endures the greatest degree of cold. In the Botanical Garden of Padua there is a high stem of considerable thickness growing in the open air, although the mean temperature of Padua is below 56° Fahrenheit.
[89]. p. 225.—“Heaths.”
We do not, in these physiognomical considerations, by any means comprehend, under the name of Heaths, the whole natural family of the Ericaceæ, which, on account of the similarity and analogy in the flowering parts of the plant, include Rhododendrum, Befaria, Gaultheria, and Escallonia; we limit ourselves to the very accordant and characteristic form of the species of Erica, including Calluna (Erica vulgaris, L.).
“Whilst in Europe Erica carnea, E. tetralix, E. cinerea, and Calluna vulgaris, cover large tracts of country, extending from the plains of Germany, and from France and England, to the extremity of Norway; Southern Africa presents the most varied assortment of species. One single species, Erica umbellata, which is indigenous in the southern hemisphere, at the Cape of Good Hope, is again found in Northern Africa, Spain, and Portugal. Erica vagans and E. arborea also belong to the opposite coasts of the Mediterranean. The former is met with in Northern Africa, in the neighbourhood of Marseilles, in Sicily and Dalmatia, and even in England; the second in Spain, Istria, Italy, and the Canaries.”[[OM]] The common heath, Calluna vulgaris (Salisbury), which is a social plant, covers large tracts from the mouth of the Scheldt to the western declivity of the Ural. Beyond the Ural both Oaks and Heaths disappear. Both are wanting in the whole of Northern Asia, and in all Siberia, as far as the Pacific. Gmelin[[ON]] and Pallas[[OO]] have expressed their astonishment at this disappearance of Calluna vulgaris; which, on the eastern declivity of the Ural chain is even more decided and more sudden than one might be led to conclude, from the words of the last-named great naturalist. Pallas merely says, “ultra Uralense jugum sensim deficit, vix in Isetensibus campis rarissime apparet, et ulteriori Sibiriæ plane deest.” Chamisso, Adolph Erman, and Heinrich Kittlitz collected Andromedas but no Calluna in Kamtschatka and on the north-west coast of America. The accurate knowledge which we at present possess of the mean temperature of different portions of Northern Asia, as well as of the distribution of annual heat throughout the different seasons, in no way explains the non-advance of the Heath to the east of the Ural. Dr. Joseph Hooker has treated with much ingenuity, in a note to his “Flora Antarctica,” of the two contrasting phenomena of the distribution of plants, “uniformity of surface accompanied by a similarity of vegetation”, and again, “instances of a sudden change in the vegetation, unaccompanied with any diversity of geological and other feature.”[[OP]] Is there an Erica in Central Asia? That which Saunders, in Turner’s “Travels to Thibet,”[[OQ]] has described in the highlands of Nepaul, besides other European plants (Vaccinium Myrtillus, and V. oxycoccus), as Erica vulgaris, is, according to the opinion communicated to me by Robert Brown, probably the Andromeda fastigiata of Wallich. The absence of Calluna vulgaris and of all species of Erica, throughout the whole of the continental part of America is an equally striking fact, since Calluna is met with in the Azores and in Iceland. It has not hitherto been found in Greenland, but it was discovered some years ago in Newfoundland. The natural family of the Ericaceæ is also almost entirely wanting in Australia, where its place is supplied by the Epacrideæ. Linnæus described only 102 species of the genus Erica, but, according to Klotzsch’s observations, this genus comprises 440 true species, after the varieties have been carefully excluded.
[90]. p. 226—“The Cactus form.”
When the natural family of the Opuntiaceæ is separated from the Grossulariaceæ (species Ribes), and is confined within the limits indicated by Kunth,[[OR]] we may regard the whole as exclusively American. I am not ignorant, that Roxburgh, in the Flora indica (inedita), mentions two species of Cactus which he regards as peculiar to the south-east of Asia, viz., Cactus indicus, and C. chinensis. Both are widely diffused, originally wild or having become so, and different from Cactus opuntia and C. Coccinellifer; but it is remarkable that this Indian plant should have no ancient Sanscrit name. The so-called Chinese Cactus has been introduced by cultivation into the island of St. Helena. Modern investigations, prosecuted at a period when a more general interest has been awakened in relation to the original distribution of plants, will unquestionably remove the doubts that have frequently been advanced against the existence of Asiatic Opuntiaceæ. We see, in a similar manner, certain vital forms appear separately in the animal world. How long did the Tapir continue to be regarded as a characteristic form of the New Continent! And yet the American Tapir is, as it were, repeated in that of Malacca (Tapirus indicus, Cuv.).
Although the Cactus form belongs, properly speaking, to the tropical regions, there are some species in the New Continent, that are indigenous to the temperate zone on the Missouri and in Louisiana; as, for instance, Cactus missuriensis and C. vivipara. Back, in his northern expedition, saw with astonishment, the banks of the Rainy Lake in lat. 48° 40′ (long. 92° 53′) entirely covered with C. Opuntia. South of the equator the Cactus does not advance further than Rio Itata (lat. 36°) and Rio Biobio (lat. 37¼°) In the part of the chain of the Andes lying within the tropics, I have found species of Cactus (C. sepium, C. chlorocarpus, C. bonplandii) on elevated plains from 9000 to upwards of 10,600 feet above the level of the sea; but in Chili, in the temperate zone, a far more strongly marked Alpine character is exhibited by Opuntia Ovallei, whose upper and lower limits have been accurately determined through barometric measurements by the learned botanist, Claude Gay. The yellow-flowering Opuntia Ovallei, which has a creeping stem, does not descend below 6746 feet, advancing as high as the line of perpetual snow; and even above it, wherever a few masses of rock remain uncovered. These little plants have been gathered at spots lying at an elevation of 13,663 feet above the level of the sea.[[OS]] Some species of Echinocactus are also true alpine plants in Chili. A counterpart to the much admired fine-haired Cactus senilis is presented by the thick-wooled Cereus lanatus, called by the natives Piscol, which has a fine red fruit. We found it near Guancabamba, in Peru, on our journey to the Amazon river. The dimensions of the Cactaceæ (a group on which the Prince of Salm-Dyck was the first to throw considerable light) present the most striking contrasts. Echinocactus Wislizeni, which has a circumference of seven feet and a half, with a height of four feet and a quarter, is only third in size, being surpassed by E. ingens, (Zucc.) and E. platyceras. (Lem.)[[OT]] The Echinocactus Stainesii attains a diameter of from two feet to two and a-half; E. visnago, belonging to Mexico, has a diameter of upwards of three feet, with a height of more than four feet, and weighs as much as from 700 to 2000 lbs.; while the Cactus nanus, which we collected near Sondorillo, in the province of Jaen, is so small and so loosely rooted in the sand, that it gets between the toes of dogs. The Melocactuses, which are full of juice even in the driest season, as the Ravenala of Madagascar (wood-leaf in the language of the country from rave, raven, a leaf, and ala, the Javanese halas, a wood), are vegetable springs, which the wild horses and mules open by stamping with their hoofs—a process in which they frequently injure themselves.[[OU]] Cactus Opuntia has spread during the last quarter of a century in a remarkable manner through Northern Africa, Syria, Greece, and the whole of Southern Europe; penetrating from the coasts of Africa far into the interior, where it associates with the native plants.
After being accustomed to see Cactuses only in our hothouses, we were astonished at the density of the woody fibres in old cactus stems. The Indians are aware that cactus wood is indestructible, and admirably adapted for oars and the thresholds of doors. There is hardly any physiognomical character of exotic vegetation that produces a more singular and ineffaceable impression on the mind of the traveller, than an arid plain densely covered with columnar or candelabra-like stems of cactuses, similar to those near Cumana, New Barcelona, Coro, and in the province of Jaen de Bracamoros.