146 THE BEAUTIES OF NATURE chap. 



The leaves of our English trees, as I have 

 already said, are so arranged as to secure the 

 maximum of light ; in very hot countries the 

 reverse is the case. Hence, in Australia, for 

 instance, the leaves are arranged not hori- 

 zontally, but vertically, so as to present, not 

 their surfaces, but their edges, to the sun. 

 One English plant, a species of lettuce, has 

 the same habit. This consideration has led 

 also to other changes. In many species the 

 leaves are arranged directly under, so as to 

 shelter, one another. The Australian species 

 of Acacia have lost their true leaves, and 

 the parts which in them we generally call 

 leaves are in reality vertically-flattened leaf 

 stalks. 



In other cases the stem itself is green, and 

 to some extent replaces the leaves. In our 

 common Broom we see an approach to this, 

 and the same feature is more marked in 

 Cactus. Or the leaves become fleshy, thus 

 offering, in proportion to their volume, a 

 smaller surface for evaporation. Of this the 

 Stonecrops, Mesembryanthemum, etc., are 

 familiar instances. Other modes of checking 



