In the preceding sketch of a “Physiognomy of Plants,” I have had principally in view three nearly allied subjects:—the absolute diversity of forms; their numerical proportion, i. e. their local predominance in the total number of species in phænogamous floras; and their geographic and climatic distribution. If we desire to rise to general views respecting organic forms, the physiognomy of plants, the study of their numerical proportions (or the arithmetic of botany),—and their geography (or the study of their zones of distribution),—cannot, as it appears to me, be separated from each other. In the study of the physiognomy of plants, we ought not to dwell exclusively on the striking contrasts presented by the larger organic forms separately considered, but we should also seek to discern the laws which determine the physiognomy of Nature generally, or the picturesque character of vegetation over the entire surface of the globe, and the impression produced on the mind of the beholder by the grouping of contrasted forms in different zones of latitude and of elevation. It is from this point of view, and with this concentration or combination of objects, that we become aware, for the first time, of the close and intimate connection between the subjects which have been treated of in the foregoing pages. We are here conducted into a field which has been as yet but little cultivated. I have ventured to follow the method first employed with such brilliant results in the Zoological works of Aristotle, and which is especially suited to lay the foundation of scientific confidence,—a method which, whilst it continually aims at generality of conception, seeks, at the same time, to penetrate the specialities of phenomena by the consideration of particular instances.

The enumeration of forms according to physiognomic diversity is, from the nature of the case, not susceptible of any strict classification. Here, as everywhere else, in the consideration of external conformation, there are certain leading forms which present the most striking contrasts: such are the groups of arborescent grasses, plants of the aloë form, the different species of cactus, palms, needle-trees, Mimosaceæ, and Musaceæ. Even a few scattered individuals of these groups are sufficient to determine the character of a district, and to produce on a non-scientific but sensitive beholder a permanent impression. Other forms, though perhaps much more numerous and preponderating in mass, may not be calculated either by the outline and arrangement of the foliage, or by the relation of the stem to the branches,—by luxuriant vigour of vegetation,—by cheerful grace,—or, on the other hand, by cheerless contraction of the appendicular organs, to produce any such characteristic impressions.

As, therefore, a “physiognomic classification,” or a division into groups from external aspect or “facies,” does not admit of being applied to the whole vegetable kingdom, so also, in such a classification, the grounds on which the division is made are quite different from those on which our systems of natural families and of plants (including the whole of the vegetable kingdom) have been so happily established. Physiognomic classification grounds her divisions and the choice of her types on whatever possesses “mass,”—such as shape, position and arrangement of leaves, their size, and the character and surfaces (shining or dull) of the parenchyma,—therefore, on all that are called more especially the “organs of vegetation,” i. e. those on which the preservation,—the nourishment and development,—of the individual depend; while systematic Botany, on the other hand, grounds the arrangement of natural families on the consideration of the organs of propagation,—those on which the continuation or preservation of the species depends. (Kunth, Lehrbuch der Botanik, 1847, Th. i. S. 511; Schleiden, die Pflanze und ihr Leben, 1848, S. 100). It was already taught in the school of Aristotle (Probl. 20, 7), that the production of seed is the ultimate object of the existence and life of the plant. Since Caspar Fried. Wolf (Theoria Generationis, § 5-9), and since our great (German) Poet, the process of development in the organs of fructification has become the morphological foundation of all systematic botany.

That study, and the study of the physiognomy of plants, I here repeat, proceed from two different points of view: the first from agreement in the inflorescence or in the delicate organs of reproduction; the second from the form of the parts which constitute the axes (i. e. the stems and branches), and the shape of the leaves, dependent principally on the distribution of the vascular fascicles. As, then, the axes and appendicular organs predominate by their volume and mass, they determine and strengthen the impression which we receive; they individualise the physiognomic character of the vegetable form and that of the landscape, or of the region in which any of the more strongly-marked and distinguished types severally occur. The law is here given by agreement and affinity in the marks taken from the vegetative, i. e. the nutritive organs. In all European colonies, the inhabitants have taken occasion, from resemblances of physiognomy (of “habitus,” “facies”), to bestow the names of European forms upon tropical plants or trees bearing very different flowers and fruits from those from which the names were originally taken. Everywhere, in both hemispheres, northern settlers have thought they found Alders, Poplars, Apple- and Olive-trees. They have been misled in most cases by the form of the leaves and the direction of the branches. The illusion has been favoured by the cherished remembrance of the trees and plants of home, and thus European names have been handed down from generation to generation; and in the slave colonies there have been added to them denominations derived from Negro languages.

The contrast so often presented between a striking agreement of physiognomy and the greatest diversity in the inflorescence and fructification,—between the external aspect as determined by the appendicular or leaf-system, and the reproductive organs on which the groups of the natural systems of botany are founded,—is a remarkable and surprising phenomenon. We should have been inclined beforehand to imagine that the shape of what are exclusively termed the vegetative organs (for example, the leaves) would have been less independent of the structure of the organs of reproduction; but in reality such a dependence only shows itself in a small number of families,—in Ferns, Grasses and Cyperaceæ, Palms, Coniferæ, Umbelliferæ, and Aroideæ. In Leguminosæ the agreement in physiognomic character is scarcely to be recognised until we divide them into the several groups (Papilionaceæ, Cæsalpinineæ, and Mimoseæ). I may name, of types which, when compared with each other, shew considerable accordance in physiognomy with great difference in the structure of the flowers and fruit, Palms and Cycadeæ, the latter being more nearly allied to Coniferæ; Cuscuta, one of the Convolvulacæ, and the leafless Cassytha, a parasitical Laurinea; Equisetum (belonging to the great division of Cryptogamia), and Ephedra, closely allied to Coniferæ. On the other hand, our common gooseberries and currants (Ribes) are so closely allied by their inflorescence to the Cactus, i. e. to the family of Opuntiaceæ, that it is only quite recently that they have been separated from it! One and the same family (that of Asphodeleæ) comprises the gigantic Dracæna draco, the common asparagus, and the Aletris with its coloured flowers. Not only do simple and compound leaves often belong to the same family, but they even occur in the same genus. We found in the high plains of Peru and New Granada, among twelve new species of Weinmannia, five with “foliis simplicibus,” and the rest with pinnate leaves. The genus Aralia shews still greater independence in the form of the leaves: “folia simplicia, integra, vel lobata, digitata et pinnata.” (Compare Kunth, Synopsis Plantarum quas in itinere collegerunt, Al. de Humboldt et Am. Bonpland, T. iii, p. 87 and 360.)

Pinnated leaves appear to me to belong chiefly to families which are in the highest grade of organic development, namely, the Polypetalæ; and among these, in the Perigynic class, to the Leguminosæ, Rosaceæ, Terebinthaceæ, and Juglandeæ; and in the Hypogynic, to the Aurantiaceæ, Cedrelaceæ, and Sapindaceæ. The beautiful doubly-pinnated leaves which form one of the principal ornaments of the torrid zone, are most frequent among the Leguminosæ, in Mimoseæ, also in some Cæsalpinieæ, Coulterias, and Gleditschias; never, as Kunth remarks, in Papilionaceæ. “Folia pinnata” and “folia composita” are never found in Gentianeæ, Rubiaceæ, and Myrtaceæ. In the morphological development presented by the abundance and variety of form in the appendicular organs of Dicotyledones, we can at present discern only a small number of general laws.