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 
offermg, in proportion to their volume, a 
smaller surface for evaporation. Of this the 
Stonecrops, Mesembryanthemum, etc., are 
familiar instances. Other modes of checking 
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