The Myrtle is a graceful plant, with stiff, shining, crowded, and generally entire and small leaves marked with dots. Myrtles impart a peculiar character to three regions of the earth, viz., to southern Europe, more especially to the islands composed of calcareous rocks and trachytic stone, which project from the basin of the Mediterranean; to the continent of New Holland, which is adorned with Eucalyptus, Metrosideros, and Leptospermum; and to an inter-tropical region in the Andes of South America, part of which is a low plain, while the remainder lies at an elevation of from 9000 to more than 10,000 feet above the level of the sea. This Alpine region, called in Quito the Paramos, is entirely covered with trees having a Myrtle-like aspect, even though they may not all belong to the Myrtaceæ. At this elevation grow Escalonia myrtilloides, E. tubar, Simplocos Alstonia, species of Myrica, and the lovely Myrtus microphylla, of which we have given a drawing in our Plantes équinoxiales, t. i. p. 21, pl. iv.; it grows on micaceous schist, at an elevation of 10,000 feet on the Paramo de Saraguru, (near Vinayacu and Alto de Pulla,) which is adorned with so many beautiful flowering Alpine plants. M. myrsinoides ascends in the Paramo de Guamani as high as 11,200 feet. By far the greater number of the 40 species of the genus Myrtus which we collected in the equinoctial zone, and of which 37 were undescribed, belong to the plains and the less elevated mountain spurs. We brought only a single species (M. xalapensis) from the mild tropical climate of the mountains of Mexico; but the Tierra templada, in the direction of the Volcano of Orizaba, no doubt possesses many yet undescribed varieties. We found M. maritima near Acapulco, on the very shore of the Pacific.
The Escalloniæ,—among which E. myrtilloides, E. tubar, E. floribunda are the ornaments of the Paramos, and remind us strongly (by their physiognomical aspect) of the myrtle-form,—formerly constituted, together with the European and South American Alpine roses (Rhododendrum and Befaria), with Clethra, Andromeda, and Gaylussacia buxifolia, the family of the Ericeæ. Robert Brown[[QW]] has arranged them in a special family, which Kunth has placed between the Philadelphiæ and Hamamelideæ. Escallonia floribunda affords by its geographical distribution one of the most striking examples of the relation existing between distance from the equator and vertical elevation above the level of the sea. I would here again borrow support from the testimony of the accurate observer, my friend Auguste de St. Hilaire.[[QX]] “MM. Humboldt and Bonpland in their expedition discovered Escallonia floribunda in 4° south lat. at an elevation of 8952 feet. I found the same plant in 21° south lat. in Brazil, which although elevated is very much less so than the Andes of Peru. This plant is of common occurrence between 24° 50′ and 25° 55′ in the Campos Geraes, and I also met with it again on the Rio de la Plata in about 35° lat., on a level with the sea.”
The group of the Myrtaceæ,—to which belong Melaleuca, Metrosideros, and Eucalyptus, commonly classed under the general denomination of Leptospermeæ,—produce partially, wherever the true leaves are supplied by phyllodia (petiole-leaves), or where the direction of the leaves is inclined towards the unexpanded petiole, a distribution of streaks of light and shade wholly unknown in our deciduous-leaved forest. We find that the earliest botanical travellers who visited New Holland were astonished at the singular effect thus produced. Robert Brown was the first to show that this phenomenon depends on the vertical direction of the expanded petioles (the phyllodia of Acacia longifolia and Acacia suaveolens), and on the circumstance, that the light, instead of falling on horizontal surfaces, passes between vertical ones.[[QY]] Morphological laws in the development of the leaves determine the peculiar character of the varying light and shade. “Phyllodia,” says Kunth, “can in my opinion merely occur in families which have compound pinnate leaves; and in fact they have as yet only been met with in Leguminosæ (in the Acacias). In Eucalyptus, Metrosideros, and Melaleuca, the leaves are simple (simplicia), and their edgewise position depends on a half-turn of the leaf-stalk (petiolus); moreover, it must be remarked, that both surfaces of the leaves are of a similar character.” In the scantily shaded forests of New Holland the optical effects here alluded to are the more frequent, since two groups of Myrtaceæ and Leguminosæ, species of Eucalyptus and Acacia, there constitute nearly one-half of all the greyish-green tree vegetation. Moreover, between the bast-layers of Melaleuca, there are formed easily soluble membranes, which force their way outwards, and by their whiteness reminds us of our birch bark.
The sphere of distribution of the Myrtaceæ is very different in the two continents. In the New Continent, and especially in its western parts, this family, according to Joseph Hooker,[[QZ]] scarcely extends beyond the parallel of 26° north lat., while in the Southern Hemisphere, there are in Chili, according to Claude Gay, ten species of Myrtle and twenty-two of Eugenia, which mixed with Proteaceæ (Embothrium and Lomatia) and with Fagus obliqua, there constitute forests. The Myrtaceæ become more frequent from the 38th degree of south lat.; in the island of Chiloe, where a metrosideros-like species (Myrtus stipularis) forms almost impenetrable underwood, which is there named Tepuales; and in Patagonia to the extremity of Tierra del Fuego in 56° lat. While in Europe the Myrtaceæ do not extend northward further than 46° lat., they penetrate in Australia, Tasmania, New Zealand and the Auckland Islands to 50½° south latitude.
[102]. p. 229—“Melastomaceæ.”
This group comprises the genera Melastoma (Fothergilla and Tococa Aub. and Rhexia (Meriana and Osbeckia), of which we have collected no less than sixty new species in tropical America alone, on both sides of the equator. Bonpland has published a splendid work on the Melastomaceæ, in two volumes, with coloured plates. There are species of Rhexia and Melastoma which ascend in the chain of the Andes, as Alpine or Paramos shrubs, to 9600 and even more than 11,000 feet above the level of the sea; as for instance Rhexia cernua, R. stricta, Melastoma obscurum, M. aspergillare, and M. lutescens.
[103]. p. 229—“The Laurel-form.”
To this form belong Laurus, Persea, the Ocoteæ, so numerous in South America, and,—on account of their physiognomic similarity,—Calophyllum, also the splendidly aspiring Mammea from the Guttiferæ.
[104]. p. 229—“How instructive to the landscape-painter would be a work which should illustrate the leading forms of vegetation.”
In order to define with more distinctness what I have here only briefly referred to, I may be permitted to incorporate the following considerations from my sketch of a history of landscape painting, and of a graphical representation of the physiognomy of plants.[[RA]]