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.