II.

Alexander Count Humboldt has submitted to the Institute a curious paper, on the laws observed in the distribution of vegetable forms over the globe.

Botany, long confined to the simple description of the external forms of plants, and their artificial classification, now presents several branches of study, which place it more on a footing with the other sciences.

Such are the distribution of vegetables, according to a natural method founded upon the whole part of their structure; physiology, which displays their internal organization; botanical geography, which assigns to each tribe of plants their height, limits, and climate.

The terms alpine plants, plants of hot countries, plants of the sea-shore, are to be found in all languages, even in those of the most savage nations on the banks of the Oronoko.

They prove that the attention of men has been constantly fixed on the distribution of vegetables, and on their connexion with the temperature of the air, the elevation of the soil, and the nature of the ground which they inhabit.

It does not require much sagacity to observe, that on the slope of the high mountains of Armenia, vegetables of a different latitude follow each in succession, like the elimates, superposed, as it were, upon each other.

The vegetables, says he, which cover the vast surface of the globe, present, when we study by natural classes or families, striking differences in the distribution of their forms.

On limiting them to the countries in which the number of the species is exactly known, and by dividing this number by that of the glumaceæ, the leguminous plants, the labiated, and the compound, we find numerical relations which form very regular series.

We see certain forms become more common, from the equator towards the pole, like the ferns, the glumaceæ, the ericineæ, and the rhododendrons.

Other forms, on the contrary, increase from the poles towards the equator, and may be considered in our hemisphere as southern forms: such are the rubiaceæ, the malvaceæ, the euphorbia, the leguminous, and the composite, plants.

Finally, others attain their maximum even in the temperate zone, and diminish also towards the equator and the poles; such are the labiated plants, the amentaceæ, the cruciferæ, and the umbelliferæ.

The grasses form in England 1-12th, in France 1-13th, in North America 1-10th, of all the phanerogamous plants.

The glumaceæ form in Germany 1-7th, in France 1-8th, in North America 1-8th, in New Holland, according to the researches of Mr.

Brown, 1-8th, of the known phanerogamous plants.

The composite plants increase a little in the northern part of the new continent; for, according to the new Flora of Pursch, there is between the parallels of Georgia and Boston 1-6th, whereas in Germany we find 1-8th, and in France 1-7th, of the total number of the species, with visible fructification.

In the whole temperate zone, the glumaceæ and the composite plants, form together, nearly one-fourth of the phanerogamous plants; the glumaceæ, the compositæ, the cruciferæ, and the leguminosæ, together,nearly one-third.

It results from these researches, that the forms of organized beings are in a mutual dependence; and that the unity of nature is such, that the forms are limited, the one after the other, according to constant laws easy of determination.

The number of vegetable species described by botanists, or existing in European herbals, extends to 44,000, of which 6000 are agamous.

In this number we had already included 3000 new phanerogamous species enumerated by M.

Bompland and myself.

France, according to M. Decandolle, possesses 3645 phanerogamous plants, of which 460 are glumaceæ, 490 composite, and 230 leguminous, &c.

In Lapland there are only 497 phanerogamous plants; among which are 124 glumaceæ, 58 composite, 14 leguminous, 23 amentaceous, &c.

Mr. Pursch has made us acquainted with 2000 phanerogamous plants which grow between the parallels of 35° and 44°; consequently, under mean annual temperatures of 16° and 7°. The flora of North America is a mixture of several floras.

The southern regions give it an abundance of malvaceæ and composite plants; the northern regions, colder than Europe, under the same parallel, furnish to this flora abundance of rhodondendrons, amentaceæ, and coniferæ.

The caryophylleæ, the umbelliferæ, and the cruciferæ, are in general more rare in North America, than in the temperate zone of the old continent.

These constant relations observed on the surface of the globe, in the plains from the equator to the pole, are again traced in the midst of perpetual snows on the summits of mountains.

We may admit, in general, that on the cordilleras of the torrid zone, the boreal forms become more frequent.

It is thus that we see prevail at Quito, on the summit of the Andes, the ericineæ, the rhododendrons, and the gramineous plants.

On the contrary, the labiatæ, the rubiaceæ, the malvaceæ, and the euphorbiaceæ, then become as rare as they are in Lapland.

But this analogy is not supported in the ferns and the composite plants.

The latter abound on the Andes, whereas the former gradually disappear when they rise above 1800 fathoms in height.

Thus the climate of the Andes resembles that of northern Europe only with respect to the mean temperature of the year.

The repartition of heat into the different seasons is entirely different, and powerfully influences the phenomena of vegetation.

In general, the forms which prevail among the alpine plants, are, according to my researches, under the torrid zone, the gramineæ (ægopogon, podosæmum, deyeuxia, avena); the compositæ (culcitium, espeletia, aster, baccharis): and the caryophylleæ (arenaria, stellaria.)

Under the temperate zone, the compositæ (senecio, leontodon, aster); the caryophylleæ (cerastium, cherleria, silene); and the cruciferæ (draba, lepidium.)

Under the frozen zone, the caryophylleæ (stellaria, alsine); the ericineæ (andromeda), and the ranunenlaceæ.

It has been long known, and it is one of the most interesting results from the geography of animals, that no quadruped, no terrestrial bird, and, as appears from the researches of M. Latreille, almost no insect, is common to the equatorial regions of the two worlds. M.

Cuvier is convinced, by precise inquiries, that this rule applies even to reptiles.

He has ascertained, that the true boa constrictor is peculiar to America; and that the boas of the old continent, were pytons.

Among the plants, we must distinguish between the agamæ and the cotyledoneæ; and by considering the latter between the monocotylodens and the dicotyledons.

There remains no doubt that many of the mosses and lichens are to be found at once in equinoctial America and in Europe.

But the case is not the same with the vascular agamæ as with the agamæ of a cellular texture.

The ferns and the lycopodiaceæ do not follow the same laws with the mosses and the lichens.The former, in particular, exhibit very few species universally to be found; and the examples cited are frequently doubtful.

As to the phanerogamous plants (with the exception of the rhizophora, the avicennia, and some other littoral plants), the law of Buffon seems to be exact with respect to the species furnished with two cotyledons.

It is absolutely false, although it has been often affirmed, that the ridges of the cordilleras of Peru, the climate of which has some analogy with the climate of France or Sweden, produce similar plants.

The oaks, the pines, the yews, the ranunculi, the rose-trees, the alchemilla, the valerians, the stellaria, the draba of the Peruvian and Mexican Andes, have nearly the same physiognomy with the species of the same genera of North America, Siberia, or Europe.

But all these alpine plants of the cordilleras, without excepting one among three or four thousand which we have examined, differ specifically from the analogous species of the temperate zone of the old continent.

In general, in that part of America situated between the tropics, the monocotyledontal plants alone, and among the latter almost solely the cyperaceæ and the gramineæ, are common to the two worlds.

These two families form an exception to the general law which we are here examining,—a law which is so important for the history of the catastrophes of our planet, and according to which, the organized beings of the equinoctial regions differ essentially in the two continents.