The Heath form[19] belongs more especially to the old world, and particularly to the African continent and islands: taking for our guides physiognomic character and general aspect, we may class under it the Epacrideæ and Diosmeæ, many Proteaceæ, and those Australian Acacias which have mere leaf-stalks instead of leaves (phyllodias). This form has some points of similarity with that of needle trees, and the partial resemblance enhances the effect of the pleasing contrast which, when these two are placed together, is afforded by the abundant bell-shaped blossoms of the heaths. 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. The declivity of the Peak of Teneriffe is the locality where I have seen them growing with the greatest luxuriance. In the countries adjoining the Baltic, and farther to the north, the aspect of this form of plants is unwelcome, as announcing sterility. Our heaths, Erica (Calluna) vulgaris, Erica tetralix, E. carnea, and E. cinerea, are social plants, and for centuries agricultural nations have combated their advance with little success. It is remarkable that the extensive genus which is the leading representative of this form appears to be almost limited to one side of our planet. Of the 300 known species of Erica only one has been discovered across the whole extent of the New Continent, from Pensylvania and Labrador to Nootka and Alashka. The Cactus form,[20], on the other hand, 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. It comprises some of the plants to which Bernardin de St. Pierre has applied the term of “vegetable fountains in the desert.” In the waterless plains of South America the animals suffering from thirst seek the melon-cactus, a spherical plant half buried in the dry sand, and encased in formidable prickles, but of which the interior abounds in refreshing juice. The stems of the columnar cactus rise to a height of 30 or 32 feet; they are often covered with lichens, and, dividing into candelabra-like branches, resemble, in physiognomy, some of the Euphorbias of Africa.