24 PHYSIOGNOMY OF PLANTS. 



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 exclu- 

 sively 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 Liliaceee 

 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 nourish in deserts 

 almost devoid of other vegetation, the Orchidese ( 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 



