While the Banana form presents us with the greatest degree of expansion, the Casuarinas and the acicular-leaved[[93]] trees exhibit the greatest contraction of the leaf-vessels. Pines, Thujas, and Cypresses constitute a northern form but rarely met with in the tropics and in some coniferæ (Dammara Salisburia), the leaves are both broad and acicular. Their evergreen foliage enlivens the gloom of the dreary winter landscape, while it proclaims to the natives of the polar regions that, although snow and ice cover the surface, the inner life of plants, like the Promethean fire, is never wholly extinct on our planet.
Besides the Orchideæ, the Pothos tribe of plants[[94]] also yields a graceful covering to the aged stems of forest trees in the tropical world, like the parasitic mosses and lichens of our own climes. Their succulent herbaceous stalks are furnished with large leaves, arrow-shaped, digitate, or elongated, and invariably furnished with thick veins. The blossoms of the Aroideæ are inclosed in spathes, by which their vital heat is increased; they are stemless, and send forth aërial roots. Pothos, Dracontium, Caladium, and Arum are all kindred forms; and the last-named extends as far as the coasts of the Mediterranean, contributing, together with succulent Tussilago (Coltsfoot), high thistles, and the Acanthus, to give a luxuriant southern character to the vegetation of Spain and Italy.
This Arum form is associated, in the torrid regions of South America, with the tropical Lianes or creeping plants[[95]], which exhibit the utmost luxuriance of vegetation in Paullinias, Banisterias, Bignonias, and Passion-flowers. Our tendrilled hops and vines remind us of this tropical form. On the Orinoco the leafless branches of the Bauhinia are often upwards of 40 feet in length, sometimes hanging perpendicularly from the summit of lofty Swieteniæ, (Mahogany trees), sometimes stretched obliquely like ropes from a mast; along these the tiger-cat may be seen climbing to and fro with wonderful agility.
The self-sustaining form of the bluish-flowered Aloe tribe[[96]] presents a marked contrast to the pliant climbing lianes with their fresh and brilliant verdure. When there is a stem it is almost branchless, closely marked with spiral rings, and surrounded by a crown of succulent, fleshy, long-pointed leaves, which radiate from a centre. The lofty-stemmed aloe does not grow in clusters like other social plants, but stands isolated in the midst of dreary solitudes, imparting to the tropical landscape a peculiar melancholy (one might almost say African) character.
To this aloe form belong, in reference to physiognomic resemblance and the impression they produce on the landscape: the Pitcairnias, from the family of the Bromeliaceæ, which in the chain of the Andes grow out of clefts in the rock; the great Pournetia pyramidata (the Atschupalla of the elevated plateaux of New Grenada); the American aloe (Agave), Bromelia Ananas and B. Karatas; those rare species of the family of the Euphorbiaceæ, which have thick, short, candelabra-like divided stems; the African aloe, and the Dragon tree, Dracæna Draco, of the family of the Asphodeleæ; and lastly the tall flowering Yucca, allied to the Liliaceæ.
While the Aloe form is characterised by an air of solemn repose and immobility, the grass form[[97]], especially as regards the physiognomy of the arborescent grasses, is expressive of buoyant lightness and flexible slenderness. In both the Indies, bamboo groves form arched and shady walks.
The smooth and often inclined and waving stem of the tropical grasses exceeds in height our alders and oaks. As far north as Italy, this form already begins, in the Arundo Donax, to raise itself from the ground, and to determine, by height as well as mass, the natural character of the country.
The form of Ferns[[98]], like that of grasses, also assumes nobler dimensions in the torrid regions of the earth, and the arborescent ferns, which frequently attain the height of above forty feet, have a palm-like appearance, although their stem is thicker, shorter, and more rough and scaly, than that of the palm. The leaf is more delicate, of a loose and more transparent texture, and sharply serrated on the margins. These colossal ferns belong almost exclusively to the tropics, but there they prefer the temperate localities. As in these latitudes diminution of heat is merely the consequence of an increase of elevation, we may regard mountains that rise 2000 or 3000 feet above the level of the sea as the principal seat of these plants. Arborescent ferns grow in South America, side by side with that beneficent tree whose stem yields the febrifuge bark, and both forms of vegetation are indicative of the happy region where reigns the genial mildness of perpetual spring.
I have now to mention the form of the Liliaceous plants[[99]], Amaryllis, Ixia, Gladiolus, and Pancratium, with their flag-like leaves and splendid blossoms, the principal home of which is Southern Africa; also the Willow form[[100]], which is indigenous in all latitudes, and is represented in the plateaux of Quito, not by the shape of its leaves, but in the form of its ramification, in Schinus Molle; also the Myrtle-form[[101]] (Metrosideros, Eucalyptus, Escallonia myrtelloides); Melastomaceæ[[102]]; and the Laurel form[[103]].
It would be an undertaking worthy of a great artist to study the character of all these vegetable groups, not in hothouses, or from the descriptions of botanists, but on the grand theatre of tropical nature. How interesting and instructive to the landscape painter[[104]] would be a work that should present to the eye accurate delineations of the sixteen principal forms enumerated, both individually and in collective contrast! What can be more picturesque than the arborescent Ferns, which spread their tender foliage above the Mexican laureloak! what more charming than the aspect of banana-groves, shaded by those lofty grasses, the Guadua and Bamboo! It is peculiarly the privilege of the artist to separate these into groups, and thus the beautiful images of nature, if we may be permitted the simile, resolve themselves beneath his touch, like the written works of man, into a few simple elements.