THE SUB-TROPICAL ZONE. 87 



few pores, defend the juices of the plants from 

 the rapid evaporation which would otherwise 

 be caused by the powerful rays of the sun. 



The ice plant is a well-known example of this 

 kind of plants, some few species of which are 

 found in our climate, for example, the common 

 house-leek. In the Island of Madeira, there is a 

 species of house-leek, which attains the form and 

 character of a tree. But some of the most 

 remarkable and peculiar plants of this zone 

 are the large and even tree-like euphorbias, 

 (Spurges,) which, with their angular, succulent 

 stems, seem to imitate the cactuses of America. 

 These plants, which are very various in form, 

 are generally of the most acrid and poisonous 

 nature, though some are possessed of exactly 

 opposite qualities, and are both innoxious and 

 nutritious. The number of species of euphor- 

 bia and its allied genera, is said to be two 

 thousand five hundred ; by far the greater part 

 of which are tropical, or sub- tropical, and very 

 few inhabit colder regions. It is believed that 

 fully half of the tropical species belong to 

 America. Euphorbia balsamifera is a native of 

 the Canary Islands, the milk of which is so in- 

 noxious and sweet, that it is thickened to jelly, 

 and eaten by the inhabitants. The cordon, 

 (Euphorbia Canariensis,) another denizen of the 

 Canaries, whose juice is as acrid as that of 

 many of the other euphorbias, is a very singular 

 plant. Its dark green, prismatic branches, six 

 inches in circumference, completely leafless, 

 and edged with pairs of short prickles, rise all 



