SOCIAL AND POLITICAL ECONOMY. 205 



animals furnishes the naturalist with a kind of 

 picture, or condensed biography, of the changes 

 through which the species or even genus may have 

 passed. Thus, with the Australian Acacias (as just 

 stated), if we raise a plant from seed, we find that 

 the first leaves developed are of the ordinary Acacia 

 kind, with slender petioles and numerous small leaf- 

 lets ; in the next the leaflets are fewer, the main 

 leaf-stalk is dilated and larger in proportion ; and 

 so on till the leaflets disappear, and the leaf-stalk 

 has been gradually transformed into the phyllodunn. 

 The process of transmutation may be seen going 

 on in such plants as Lathyrus alata^ which bears 

 both pJiyllodes and perfect leaves on the same plants 

 and at the same time. 



The Cacti, and those of the Euphorbiacece which 

 have so singularly mimicked the Cactus -shape and 

 outlines, are ah/ays armed with sharp and dangerous 

 thorns to defend the succulent tissues from thirsty 

 and hungry mammals. These sharp, tough spines 

 are in reality leaves, whose original duty has been 

 put off for the more important one of defence. And 

 the functions formerly performed by such trans- 

 formed leaves are now undertaken by the epidermis 

 of the green trunk and angular stems, which are 

 covered with stomata^ just as the under surfaces of 

 leaves usually are. In short, the leaf- functions 

 are thus carried out by the whole exposed surfaces 

 of the plant. All those Orchids (chiefly tropical) 



