a CAOTAOEOnS PLANTS. 



protected by a thick impervious leather-like epidermis, which effectually 

 enables them to withstand the rapid evaporation constantly proceeding 

 in such regions. It is not easy to exactly reproduce these conditions 

 artificially when cultivating the plants in our northern climate ; we can, 

 however, insure a sufficiently high temperature with the requisite aridity 

 of soil or atmosphere, and this seems all that is needed, though undoubtedly 

 the maturing effect of a roasting sun is missed to some extent. For these 

 really tropical species a special house is required to give the cultivator an 

 opportunity of growing them to the best advantage, as when included in 

 a mixed collection of plants they are usually exposed to a much greater 

 humidity than is beneficial to them, and they consequently flower less 

 freely. This applies especially to the majority of the species of Echino- 

 cactus, Melocactus, Opuntia, and Cereus, though in the latter there 

 are several exceptions, as some require the moist atmosphere of an 

 ordinary stove. Nearly all the family need a free exposure to the sun ; 

 but even amongst the tropical species there are exceptions to this rule, 

 for Ehipsalis and Bpiphyllum, being epiphytal in habit, succeed well 

 in moderate shade, and can indeed be associated with such moisture- 

 loving plants as Orchids and Ferns. 



In ascending the mountains of central and northern America, where 

 the Cacteffi abound, some of the wandering species of most of the genera 

 are found similarly happy in much lower temperatures, until at an alti- 

 tude of 8 to 10,000 feet in the Rocky Mountains some forms of Opuntia 

 and Echinocactus still constitute a portion of the vegetation, and are 

 ^^equently exposedto_frggts_and snow. It will thus be seen that in a 

 broadrsense tlJeT^actee may for cultural purposes be divided into three 

 groups : — 1st, the tropical species from the lowlands and plains, extending 

 for a short distance up the mountains ; 2nd, the intermediate species, 

 which are found on the extreme northern limits of their distribution, and 

 which rise to a considerable height in the mountains, but still below the 

 frost line ; and third, those which inhabit the highest elevations. The 

 two first would respectively require the temperature of a dry stove and a 

 greenhouse, while the remaining group includes those which have been 

 found to be hardy in England. 



Except where very large collections are formed and every effort is 

 made to imitate the natural conditions, houses cannot be specially de- 

 voted to each of the two former groups, and the plants are so extremely 

 accommodating that they will apparently conform themselves to any 

 reasonable treatment. Wherever these plants are made a speciality one 

 house at least can be appropriated to them, and this is really all that is 

 necessary, for the species from the tropical regions can be placed at the 

 warmest end, the intermediate and cool species being accommodated ia 



