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.
While the above-mentioned plants flourish in deserts almost devoid of other vegetation, the Orchideæ[21] 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 colours and strange and curious shape, sometimes resembling that of
