[29] p. 28.—“Liliaceæ.”
The principal seat of this form is Africa, where it is both most varied and most abundant, and where these beautifully flowering plants are assembled in masses and determine the aspect and character of the country. The New Continent does, indeed, also possess superb Alstromeriæ and species of Pancratium, Hæmanthus, and Crinum (we augmented the first-named of these genera by nine, and the second by three species); but these American Liliaceæ grow dispersed, and are less social than our European Irideæ.
[30] p. 28.—“Willow Form.”
Of the leading representative of this form, the Willow itself, 150 different species are already known. They are spread over the northern hemisphere from the Equator to Lapland. They appear to increase in number and diversity of form between the 46th and 70th degrees of north latitude, and especially in the part of north of Europe where the configuration of the land has been so strikingly indented by early geological changes. Of Willows as tropical plants I am acquainted with ten or twelve species, which, like the willows of the southern hemisphere, are deserving of particular attention. As Nature seems as it were to take pleasure in multiplying certain forms of animals, for example Anatidæ (Lamellirostres) and Columbæ, in all the zones of the earth; so are Willows, the different species of Pines, and Oaks, no less widely disseminated: the latter (oaks) being always alike in their fruit, though much diversified in the forms of their leaves. In Willows, the similarity of the foliage, of the ramification, and of the whole physiognomic appearance, in the most different climates, is unusually great,—almost greater than even in Coniferæ. In the southern part of the temperate zone of the northern hemisphere the number of species of willows decreases considerably, yet (according to the Flora atlantica of Desfontaines) Tunis has still a species of its own resembling Salix caprea; and Egypt reckons, according to Forskäl, five species, from the catkins of whose male flowers a medicine much employed in the East, Moie chalaf (aqua salicis) is obtained by distillation. The Willow which I saw in the Canaries is also, according to Leopold von Buch and Christian Smith, a peculiar species, common however to that group and to the Island of Madeira,—S. canariensis. Wallich’s Catalogue of the plants of Nepaul and of the Himalaya cites from the Indian sub-tropical zone thirteen species, partly described by Don, Roxburgh, and Lindley. Japan has its indigenous willows, one of which, S. japonica (Thunb.) is also found as a mountain plant in Nepaul.
Previous to my expedition, the Indian Salix tetrasperma was the only known intertropical species, so far as I am aware. We collected seven new species, three of which were from the elevated plains of Mexico, and were found to extend to an elevation of 8000 (about 8500 English) feet above the level of the sea. At still greater elevations,—for example, on the mountain plains situated between 12000 and 14000 feet, (about 12790 and 14920 English,) which we often visited,—we did not find, either in the Andes of Mexico or in those of Quito and Peru, anything which could recall the small creeping alpine willows of the Pyrenees, the Alps, and Lapland (S. herbacea, S. lanata, and S. reticulata). In Spitzbergen, where the meteorological conditions have much analogy with those of the Swiss and Scandinavian snow-mountains, Martins described two dwarf willows, of which the small woody stems and branches creep on the ground, and which lie so concealed in the turf-bogs that their small leaves are only discovered with difficulty under the moss. The species found by me in Peru in 4° 12´ S. latitude, near Loxa, at the entrance of the forests where the best Cinchona bark is collected, and described by Willdenow as Salix humboldtiana, is the one which is most widely distributed in the western part of South America. A sea-shore species, S. falcata, which we found on the sandy coast of the Pacific, near Truxillo, is, according to Kunth, probably only a variety of the above; and possibly the fine and often pyramidal willow which accompanied us along the banks of the Magdalena, from Mahates to Bojorque, and which, according to the report of the natives, had only extended so far within a few years, may also be identical with Salix humboldtiana. At the confluence of the Rio Opon with the Magdalena, we found all the islands covered with willows, many of which had stems 64 English feet high, but only 8 to 10 inches in diameter. (Humboldt and Kunth, Nova Gen. Plant. T. ii. p. 22, tab. 99.) Lindley has made us acquainted with a species of Salix from Senegal, and therefore in the African equinoctial zone. (Lindley, Introduction to the Natural System of Botany, p. 99.) Blume also found two species of Salix near the equator, in Java: one wild and indigenous, S. tetrasperma; and another cultivated, S. sieboldiana. From the southern temperate zone I know only two willows described by Thunberg, (S. hirsuta and S. mucronata); they grow by the side of Protea argentea (which has itself very much the physiognomy of a willow), on the banks of the Orange River, and their leaves and young shoots form the food of the hippopotamus. Willows are entirely wanting in Australia and the neighbouring islands.
[31] p. 29.—“Myrtaceæ.”
An elegant form, with stiff, shining, thickly set, generally unindented, small leaves, studded with pellucid dots. Myrtaceæ give a peculiar character to three districts of the earth’s surface,—the South of Europe, particularly the calcareous and trachytic islands which rise above the surface of the Mediterranean;—the continent of New Holland, adorned with Eucalyptus, Metrosideros, and Leptospermum;—and an intertropical region, part of which is low, and part from nine to ten thousand feet high (about 9590 to 10660 English), in the Andes of South America. This mountain district, called in Quito the district of the Paramos, is entirely covered with trees which have a myrtle-like aspect and character, even though they may not all belong to the natural family of Myrtaceæ. Here, at the above-named elevation, grow the Escallonia myrtilloides, E. tubar, Simplocos alstonia, some species of Myrica, and the beautiful Myrtus microphylla which we have figured in the Plantes équinoxiales, T. i. p. 21, Pl. iv. We found it growing on mica slate, and extending to an elevation of more than ten thousand English feet, on the Paramo de Saraguru, near Vinayacu and Alto de Pulla, which is adorned with so many lovely alpine flowering plants. Myrtus myrsinoides even extends in the Paramo de Guamani up to 10500 (11190 English) feet. Of the 40 species of the Genus Myrtus which we collected in the equinoctial zone, and of which 37 were undescribed, much the greater part belonged, however, to the plains and lower mountains. From the mild tropical mountain climate of Mexico we brought back only a single species (Myrtus xalapensis); but the Tierra templada, towards the Volcano of Orizaba, must no doubt contain several more. We found M. maritima near Acapulco, quite on the sea-coast of the Pacific.
The Escallonias,—among which E. myrtilloides, E. tubar, and E. floribunda, are the ornament of the Paramos, and by their physiognomy remind the beholder strongly of the myrtle-form,—once constituted, in combination with the European and South American Alp-roses (Rhododendrum and Befaria), and with Clethra, Andromeda, and Gaylussaccia buxifolia, the family of Ericeæ. Robert Brown (see the Appendix to Franklin’s Narrative of a Journey to the Shores of the Polar Sea, 1823, p. 765), has raised them to the rank of a separate family, which Kunth places between Philadelpheæ and Hamamelideæ. The Escallonia floribunda offers in its geographical distribution one of the most striking examples, in the habitat of the plant, of proportion between distance from the equator and vertical elevation above the level of the sea. In making this statement I again support myself on the authority of my acute and judicious friend Auguste de St.-Hilaire (Morphologie végétale, 1840, p. 52):—“Messieurs de Humboldt et Bonpland ont découvert dans leur expédition l’Escallonia floribunda à 1400 toises par les 4° de latitude australe. Je l’ai retrouvé par les 21° au Brésil dans un pays élevé, mais pourtant infiniment plus bas que les Andes du Pérou: il est commun entre les 24°.50´ et les 25°.55´ dans les Campos Geræs, enfin je le revois au Rio de la Plata vers les 35°, au niveau même l’ocean.”
Trees belonging the group of Myrtaceæ,—to which Melaleuca, Metrosideros, and Eucalyptus belong in the sub-division of Leptospermeæ,—produce partially, either where the leaves are replaced by phyllodias (leaf-stalk leaves), or by the peculiar disposition or direction of the leaves relatively to the unswollen leaf-stalk, a distribution of stripes of light and shade unknown in our forests of round-leaved trees. The first botanical travellers who visited New Holland were struck with the singularity of the effect thus produced. Robert Brown was the first to show that this strange appearance arose from the leaf-stalks (the phyllodias of the Acacia longifolia and A. suaveolens) being expanded in a vertical direction, and from the circumstance that the light instead of falling on horizontal surfaces, falls on and passes between vertical ones. (Adrien de Jussieu, Cours de Botanique, p. 106, 120, and 700; Darwin, Journal of Researches, 1845, p. 433). Morphological laws in the development of the leafy organs determine the peculiar character of the effects produced, the outlines of light and shade. “Phyllodias,” says Kunth, “can, according to my view, only occur in families which have compound pinnated leaves; and in point of fact they have as yet only been found in Leguminosæ, (in Acacias). In Eucalyptus, Metrosideros, and Melaleuca, the leaves are simple (simplicia), and their edgewise position arises from a half turn or twist of the leaf-stalk (petiolus); it should be remarked at the same time that the two surfaces of the leaves are similar.” In the comparatively shadeless forests of New Holland the optical effects here alluded to are the more frequent, as two groups of Myrtaceæ and Leguminosæ, species of Eucalyptus and of Acacia, constitute almost the half of all the greyish green trees of which those forests consist. In addition to this, in Melaleuca there are formed between the layers of the inner bark easily detached portions of epidermis which press outwards, and by their whiteness remind the European of our birch bark.
The distribution of Myrtaceæ is very different in the two continents. In the New Continent, and especially in its western portion, it scarcely extends beyond the 26th parallel of north latitude, according to Joseph Hooker (Flora antarctica, p. 12); while in the Southern Hemisphere, according to Claude Gay, there are in Chili 10 species of Myrtus and 22 species of Eugenia, which, intermixed with Proteaceæ (Embothrium and Lomatia), and with Fagus obliqua, form forests. The Myrtaceæ become more abundant beyond 38° S. lat.,—in the Island of Chiloe, where a Metrosideros-like species of Myrtus (Myrtus stipularis) forms almost impenetrable thickets under the name of Tepuales; in Patagonia; and in Fuegia to its extremity in 56½° S. lat. In the Old Continent they prevail in Europe as far as the 46th parallel of North latitude: in Australia, Tasmania, New Zealand, and the Auckland Islands, they advance to 50½° South latitude.