Note 31 — PHYSIOGNOMY OF PLANTS.

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.