In the foregoing sketch of a “Physiognomy of Plants,” I have endeavoured to keep in view three nearly allied subjects,—the absolute diversity of forms; their numerical relations, i.e. their local preponderance in the whole number of phanerogamic floras; and their geographical and climatic distribution. If we would rise to a general view regarding vital forms;—the physiognomy, the study of the numerical relations (the arithmetic of botany), and the geography of plants (the study of the local zones of distribution), cannot, as it seems to me, be separated from one another. The study of the physiognomy of plants must not be exclusively directed to the consideration of the striking contrasts of form which the larger organisms present, when considered separately; but it must rise to the recognition of the laws which determine physiognomy of nature generally, the picturesque character of vegetation over the whole surface of the earth, and the vivid impression produced by the grouping of contrasted forms in different zones of latitude and elevation. It is when concentrated into this focus that we first clearly perceive the close and intimate connection existing between the subjects treated of in the preceding pages. We have here entered upon a field of inquiry hitherto but little cultivated. I have ventured to follow the method first propounded with such brilliant results in Aristotle’s zoological works, and which is so especially adapted to establish scientific confidence,—a method in which the incessant effort to arrive at a generalisation of ideas supported by individual illustrations, is associated with an endeavour to penetrate to the specialities of phenomena.

The enumeration of forms is, from the physiognomical difference of their nature, incapable of any strict classification. Here, as everywhere in the consideration of external forms, there are certain main types which present the strongest contrasts,—as the groups of the Arborescent Grasses, the Aloe form and the species of Cactus, Palms, Acicular-leaved trees, Mimosaceæ, and Bananas. Even scantily dispersed individuals belonging to these groups determine the character of a district, and produce a lasting impression on the mind of the unscientific but susceptible beholder. Other forms, perhaps more numerous and preponderating, may not appear equally marked either by the shape or position of the leaves; the relation of the stem to the branches, luxuriant vigour, animation, and grace; or even by the melancholy contraction of the leaf-organs.

As, therefore, a physiognomical classification, or a distribution into groups according to external appearance, does not admit of being applied to the whole vegetable kingdom collectively, the basis on which such a classification should be grounded must necessarily be wholly different from that which has been so happily chosen for the establishment of our comprehensive systems of the natural families of plants. Vegetable physiognomy grounds its divisions and the choice of its types on all that possesses mass,—as the stem, branches, and appendicular organs (the form, position, and size of the leaf, the character and brilliancy of the parenchyma), and consequently on all that is now included under the special term, the organs of vegetation, and on which depend the preservation (nourishment and development) of the individual; while systematic botany, on the other hand, bases the arrangement of the natural families of plants on a consideration of the organs of propagation, on which depends the preservation of the species.[[RC]] It was already taught in the school of Aristotle,[[RD]] that the generation of seed is the ultimate aim of the being and life of a plant. The process of development in the organs of fructification has become, since Caspar Fried. Wolf,[[RE]] and our great poet Goëthe, the morphological basis of all systematic botany.

This science and that also of vegetable physiognomy proceed, I would here again observe, from two different points of view; the former depending upon an accordance in the inflorescence and in the reproduction of the delicate sexual organs; the latter on the conformation of the parts constituting the axes (the stem and branches) and on the outline of the leaves, which are mainly determined by the distribution of the vascular bundles. As, moreover, the stem and branches, together with their appendicular organs, predominate by mass and volume, they determine and strengthen the impression we receive, while they individualize the physiognomical character of the vegetation, as well as that of the landscape or the zone in which some distinguished types occur. The law is here expressed by the accordance and affinity in the marks appertaining to the vegetative, i.e. the nutritient organs. In all European colonies the inhabitants have been led by resemblances of physiognomy (habitus, facies) to apply the names of European forms to certain tropical plants, which bear wholly different flowers and fruits from the genera to which these designations originally referred. Everywhere in both hemispheres, the northern settler has believed he could recognise Alders, Poplars, Apple and Olive trees; being misled for the most part by the form of the leaves and the direction of the branches. The charm associated with the remembrance of native forms has strengthened the illusion, and European names of plants have thus been perpetuated from generation to generation in the slave colonies, where they have been further enriched by denominations borrowed from the negro languages.

A remarkable phenomenon is presented by the contrast frequently observed to arise from a striking accordance in physiognomy, coupled with the greatest difference in the organs of inflorescence and fructification—between the external form as determined by the appendicular or leaf-system, and the sexual organs on which are based the various groups of the natural systems of botany. One would be disposed à priori to believe that the aspect of vegetative organs (leaves) exclusively so called, must depend upon the structure of the organs of reproduction, but this dependence has only been observed in a very small number of families, as Ferns, Grasses, Cyperaceæ, Palms, Coniferæ, Umbelliferæ, and Aroideæ. In the Leguminosæ this accordance between the physiognomical character and the inflorescence can scarcely be recognized, excepting where they are separated into groups (as Papilionaceæ, Cæsalpinineæ, and Mimosaceæ.) The types which exhibit, when compared together, a very different structure of inflorescence and fructification, notwithstanding external accordance in physiognomy, are Palms and Cycadeæ, the latter being most nearly allied to the Coniferæ; Cucusta, belonging to the Convolvulaceæ, and the leafless Cassytha, a parasitical Laurinea; Equisetum (from the division of the Cryptogamia) and Ephedra (a coniferous tree). The Grossulareæ (Ribes) are so nearly allied by their efflorescence to Cactuses, i. e. the family of the Opuntiaceæ, that it is only very lately that they have been separated from them! One common family (that of the Asphodeleæ) comprises the gigantic tree, Dracœna Draco, the Common Asparagus, and the coloured flowering Aletris. Simple and compound leaves frequently belong not only to the same family, but even to the same genus. We found in the elevated plateaux of Peru and New Granada among twelve new species of Weinmannia, five with simple, and the remainder with pinnate leaves. The genus Aralia exhibits yet greater independence in the leaf-form, which is either simple, entire, lobed, digitate, or pinnate.[[RF]]

Pinnate leaves appear to me to belong especially to those families which occupy the highest grade of organic development, as for instance, the Polypetalœ; among perigynic plants, the Leguminosæ, Rosaceæ, Terebinthaceæ, and Juglandeæ; among hypogynic plants the Aurantiaceæ, Cedrelaceæ, and Sapindaceæ. The elegant form of the doubly pinnate leaf, which constitutes so great an adornment of the torrid zone, is most frequently met with among the Leguminosæ; among the Mimosaceæ, and also among some Cæsalpinias, Coulterias and Gleditschias; but never, as Kunth has observed, among the Papilionaceæ.

The form of pinnate, and more especially of compound leaves, is unknown in Gentianeæ, Rubiaceæ, and Myrtaceæ. In the morphological development presented by the richness and varied aspect of the appendicular organs of dicotyledons, we are only able to recognize a very small number of general laws.

ON THE
STRUCTURE AND MODE OF ACTION
OF
VOLCANOS
IN DIFFERENT PARTS OF THE EARTH.

(This Memoir was read at a Public Meeting of the Academy, at Berlin, on the 24th January, 1823.)

When we consider the influence exerted on the study of nature during the last few centuries, by the extension of geographical knowledge and by means of scientific expeditions to remote regions of the earth, we are at once made sensible of the various character of this influence, according as the investigations have been directed to the forms of the organic world, the study of the inorganic crust of the earth, or to the knowledge of rocks, their relative ages, and their origin. Different vegetable and animal developments exist in every division of the earth, whether it be on the plains, where, on a level with the sea, the temperature varies with the latitude and with the various inflections of the isothermal lines, or on the steep declivity of mountain ranges, warmed by the direct rays of the sun. Organic nature imparts to every region of the globe its own characteristic physiognomy. But this does not apply to the inorganic crust of the earth divested of its vegetable covering, for everywhere, in both hemispheres, from the equator to the poles, the same rocks are found grouped with some relation to each other, either of attraction or repulsion. In distant lands, surrounded by strange forms of vegetation, and beneath a sky beaming with other stars than those to which his eye had been accustomed, the mariner often recognises, with joyful surprise, argillaceous schists and rocks familiar to him in his native land.