Next to the last-mentioned form of which the Pothos and Arum are representatives, I place a form with which, in the hottest parts of South America, it is frequently associated,--that of the tropical twining rope-plants, or Lianes,[25] which display in those regions, in Paullinias, Banisterias, Bignonias, and Passifloras, the utmost vigour of vegetation. It is represented to us in the temperate latitudes by our twining hops, and by our grape vines. On the banks of the Orinoco the leafless branches of the Bauhinias are often between 40 and 50 feet long: sometimes they hang down perpendicularly from the high top of the Swietenia, and sometimes they are stretched obliquely like the cordage of a ship: the tiger-cats climb up and descend by them with wonderful agility. In strong contrast with the extreme flexibility and fresh light-coloured verdure of the climbing plants, of which we have just been speaking, are the rigid self-supporting growth and bluish hue of the form of Aloes,[26] which, instead of pliant stems and branches of enormous length, are either without stems altogether, or have branchless stems. The leaves, which are succulent, thick, and fleshy, and terminate in long points, radiate from a centre and form a closely crowded tuft. The tall-stemmed aloes are not found in close clusters or thickets like other social or gregarious plants or trees; they stand singly in arid plains, and impart thereby to the tropical regions in which they are found a peculiar, melancholy, and I would almost venture to call it, African character. Taking for our guides resemblance in physiognomy, and influence on the impression produced by the landscape, we place together under the head of the Aloe form, (from among the Bromeliaceæ) the Pitcairnias, which in the chain of the Andes grow out of clefts in the rocks; the great Pournetia pyramidata, (the Atschupalla of the elevated plains of New Granada); the American Aloe, (Agave); Bromelia aranas and B. karatas; from among the Euphorbiaceæ the rare species which have thick short candelabra-like divided stems; from the family of Asphodeleæ the African Aloe and the Dragon tree, (Dracæna draco); and lastly, from among the Liliaceæ, the tall flowering Yucca. If the Aloe form is characterised by an almost mournful repose and immobility, the form of Gramineæ,[27] especially the physiognomy of arborescent grasses, is characterised, on the contrary, by an expression of cheerfulness and of airy grace and tremulous lightness, combined with lofty stature. Both in the East and West Indies groves of Bamboo form shaded over-arching walks or avenues. The smooth polished and often lightly-waving and bending stems of these tropical grasses are taller than our alders and oaks. The form of Gramineæ begins even in Italy, in the Arundo donax, to rise from the ground, and to determine by height as well as mass the natural character and aspect of the country.