247 



very strange, almost cartilaginous cells, the walls of which are often 

 traversed by elegant little canals. Its thickness varies in different 

 species, and it is thickest, and therefore most impenetrable, in Melo- 

 cacti, which grow in the driest and hottest regions, while it is least 

 remarkable in the species of Rhipsalis, which are parasites on the 

 trees of the damp Brazilian forests." — p. 221. 



As a matter of course, the Cactaceae could not be treated on without 

 some allusion to their various economical uses. Almost all bear an 

 edible fruit, which, as Schleiden well observes, may be looked upon 

 as " a nobler form of our native gooseberry and currant, to which also 

 they are the nearest allies in a botanical point of view." The old 

 dead woody stems of the torch-thistles {Cereus) are, as their name 

 implies, used as torches ; and they are carried up the Cordilleras on 

 mules to serve as beams, posts, and door-sills to the houses. The 

 Opuntias are used in Mexico and other parts both of America and 

 Europe to form hedges : the spines of Opuntia Tuna are said to be 

 so large and strong as to kill the buffaloes by the inflammation follow- 

 ing wounds inflicted by them ; and it was this species, planted in a 

 triple row, which formed the boundary line between the English and 

 French in the Island of St. Christopher. But it is in a mercantile 

 view as the supporters of the cochineal insect {Coccus Cacti), that 

 these plants have perhaps attained the highest importance. Hum- 

 boldt has stated that the importation of cochineal from Oaxaca alone 

 is valued at £500,000; the pound costing about 30s., and containing 

 some 70,000 insects, which will give an idea of the enormous num- 

 ber of insects, and the great extent of this peculiar kind of culture. 



The deformed and shapeless forms of the stems of these plants is 

 abundantly compensated by the splendour of their flowers, none of 

 which yield the palm to the splendid blossom of the night-flowering 

 Cereus {Cereus grandiflorus), about eight inches in diameter, which, 

 with their vanilla-like scent, unfold in the evening, are fully expanded 

 about midnight, and by morning faded never to revive again. 



In the lecture on " The Geography of Plants," the laws which re- 

 gulate the distribution of the vegetable kingdom are discussed in the 

 same j:>opular style as the other branches of the science. In con- 

 nexion with this subject lie, side by side, a soluble and an insoluble 

 problem ; the one soluble, because it can be stated definitely as " the 

 Dependence of the Distribution of Plants on the Physical Conditions 

 of the Earth ;" the other insoluble, " because no definite proposition 

 can be laid down which the inquirer may apply himself to elucidate." 

 To the first belong such facts as are explicable upon a consideration 



