In Using This Expression, It Is Tacitly Admitted, That Under The Influence Of Certain Temperatures, Certain Vegetable Forms Must Necessarily Be Developed.

Such a supposition, however, taken in all its generality, is not strictly accurate.

The pines of Mexico are wanting on the Cordilleras of Peru. The Silla of Caracas is not covered with the oaks which flourish in New Grenada at the same height. Identity of forms indicates an analogy of climate; but in similar climates the species may be singularly diversified.

The charming rhododendron of the Andes (the befaria) was first described by M. Mutis, who observed it near Pamplona and Santa Fe de Bogota, in the fourth and seventh degree of north latitude. It was so little known before our expedition to the Silla, that it was scarcely to be found in any herbal in Europe. The learned editors of the Flora of Peru had even described it under another name, that of acunna. In the same manner as the rhododendrons of Lapland, Caucasus, and the Alps* (* Rhododendron lapponicum, R. caucasicum, R. ferrugineum, and R. hirsutum.) differ from each other, the two species of befaria we brought from the Silla* (* Befaria glauca, B. ledifolia.) are also specifically different from that of Santa Fe and Bogota.* (* Befaria aestuans, and B. resinosa.) Near the equator the rhododendrons of the Andes (Particularly B. aestuans of Mutis, and two new species of the southern hemisphere, which we have described under the name of B. coarctata, and B. grandiflora.) cover the mountains as far as the highest paramos, at sixteen and seventeen hundred toises of elevation. Advancing northward, on the Silla de Caracas, we find them much lower, a little below one thousand toises. The befaria recently discovered in Florida, in latitude 30 degrees, grows even on hills of small elevation. Thus in a space of six hundred leagues in latitude, these shrubs descend towards the plains in proportion as their distance from the equator augments. The rhododendron of Lapland grows also at eight or nine hundred toises lower than the rhododendron of the Alps and the Pyrenees. We were surprised at not meeting with any species of befaria in the mountains of Mexico, between the rhododendrons of Santa Fe and Caracas, and those of Florida.

In the small grove which crowns the Silla, the Befaria ledifolia is only three or four feet high. The trunk is divided from its root into a great many slender and even verticillate branches. The leaves are oval, lanceolate, glaucous on their inferior part, and curled at the edges. The whole plant is covered with long and viscous hairs, and emits a very agreeable resinous smell. The bees visit its fine purple flowers, which are very abundant, as in all the alpine plants, and, when in full blossom, they are often nearly an inch wide.

The rhododendron of Switzerland, in those places where it grows, at the elevation of between eight hundred and a thousand toises, belongs to a climate, the mean temperature of which is +2 and-1 degrees, like that of the plains of Lapland. In this zone the coldest months are-4, and-10 degrees: the hottest, 12 and 7 degrees. Thermometrical observations, made at the same heights and in the same latitudes, render it probable that, at the Pejual of the Silla, one thousand toises above the Caribbean Sea, the mean temperature of the air is still 17 or 18 degrees; and that the thermometer keeps, in the coolest season, between 15 and 20 degrees in the day, and in the night between 10 and 12 degrees. At the hospital of St. Gothard, situated nearly on the highest limit of the rhododendron of the Alps, the maximum of heat, in the month of August at noon, in the shade, is usually 12 or 13 degrees; in the night, at the same season, the air is cooled by the radiation of the soil down to +1 or-1.5 degrees. Under the same barometric pressure, consequently at the same height, but thirty degrees of latitude nearer the equator, the befaria of the Silla is often, at noon, in the sun, exposed to a heat of 23 or 24 degrees. The greatest nocturnal refrigeration probably never exceeds 7 degrees. We have carefully compared the climate, under the influence of which, at different latitudes, two groups of plants of the same family vegetate at equal heights above the level of the sea. The results would have been far different, had we compared zones equally distant, either from the perpetual snow, or from the isothermal line of 0 degrees.* (* The stratum of air, the mean temperature of which is 0 degrees, and which scarcely coincides with the superior limit of perpetual snow, is found in the parallel of the rhododendrons of Switzerland at nine hundred toises; in the parallel of the befarias of Caracas, at two thousand seven hundred toises of elevation.)

In the little thicket of the Pejual, near the purple-flowered befaria, grows a heath-leaved hedyotis, eight feet high; the caparosa,* which is a large arborescent hypericum (* Vismia caparosa (a loranthus clings to this plant, and appropriates to itself the yellow juice of the vismia); Davallia meifolia, Heracium avilae, Aralia arborea, Jacq., and Lepidium virginicum. Two new species of lycopodium, the thyoides, and the aristatum, are seen lower down, near the Puerto de la Silla.); a lepidium, which appears identical with that of Virginia; and lastly, lycopodiaceous plants and mosses, which cover the rocks and roots of the trees. That which gives most celebrity in the country to the little thicket, is a shrub ten or fifteen feet high, of the corymbiferous family. The Creoles call it incense (incienso).* (* Trixis nereifolia of M. Bonpland.) Its tough and crenate leaves, as well as the extremities of the branches, are covered with a white wool. It is a new species of Trixis, extremely resinous, the flowers of which have the agreeable odour of storax. This smell is very different from that emitted by the leaves of the Trixis terebinthinacea of the mountains of Jamaica, opposite to those of Caracas.

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