The form of Malvaceæ and Bombaceæ, represented by Ceiba, Cavanillesia, and the Mexican hand-tree Cheirostemon, has enormously thick trunks; large, soft, woolly leaves, either heart-shaped or indented; and superb flowers, frequently of a purple or crimson hue. It is to this group of plants that the baobab, or monkey bread-tree (Adansonia digitata), belongs, which, with a very moderate elevation, has a diameter of 32 English feet, and is probably the largest and most ancient organic monument on our planet. In Italy the Malvaceæ already begin to impart to the vegetation a peculiar southern character. The delicately pinnated foliage of the Mimosa form, of which Acacia, Desmanthus, Gleditschia, Porleria, and Tamarindus are important members, is entirely wanting in our temperate zone in the Old Continent, though found in the United States, where, in corresponding latitudes, vegetation is more varied and vigorous than in Europe. The umbrella-like arrangement of the branches, resembling that seen in the stone-pine in Italy, is very frequent among the Mimosas. The deep blue of the tropic sky seen through their finely divided foliage has an extremely picturesque effect. The heath form belongs more especially to the African continent and islands. Arborescent heaths, like some other African plants, extend to the northern shores of the Mediterranean; they adorn Italy and the cistus-covered grounds of the south of Spain. In the countries adjoining the Baltic, and further to the north, the aspect of this form of plants is unwelcome as announcing sterility. The cactus form is almost exclusively American. Sometimes spherical, sometimes articulated or jointed, and sometimes assuming the shape of tall, upright polygonal columns resembling the pipes of an organ, this group presents the most striking contrast to those of Liliaceæ and bananas. While the above-mentioned plants flourish in deserts almost devoid of vegetation, the Orchideæ enliven the clefts of the wildest rocks and the trunks of tropical trees blackened by excess of heat. This form (to which the vanilla belongs) is distinguished by its bright green succulent leaves, and by its flowers of many colors and strange and curious shape, sometimes resembling that of winged insects, and sometimes that of the birds which are attracted by the perfume of the honey vessels. Such is their number and variety that, to mention only a limited district, the entire life of a painter would be too short for the delineation of all the magnificent Orchideæ which adorn the recesses of the deep valleys of the Andes of Peru. The Casuarina form, leafless, like almost all species of cactus, consists of trees with branches resembling the stalks of our Equisetums. It is found only in the islands of the Pacific and in India, but traces of the same singular rather than beautiful type are seen in other parts of the world.