50 THE STORY OF THE PLANTS. 



green-stuff, or chlorophyll, can perform the func- 

 tions of a leaf. In very dry or desert places, 

 leaves would be useless, because their flat and 

 exposed blades would allow the water within to 

 evaporate too readily. Hence most desert plants, 

 like the cactuses, and many kinds of acacias and 

 euphorbias, have no true leaves at all ; in their 

 place they have thick and fleshy stems, often very 

 leaf-like in shape, and curiously jointed. These 

 stems are covered with a thick, transparent skin 

 or epidermis, to resist evaporation, and are pro- 

 tected by numerous stinging hairs or spines, 

 which serve to keep off the attacks of animals. 

 Stems of this type are used as reservoirs of water, 

 which the plant sucks up during the infrequent 

 rains ; and as they contain chlorophyll, like leaves, 

 they serve in just the same way as swallowersand 

 digesters of carbonic acid. 



Many other plants which live in. dry or sandy 

 places, like our common English stone-crops, do 

 not go quite as far as the cactuses, but have thick 

 and fleshy leaves on thick and fleshy stems, to 

 prevent evaporation. As a general rule, indeed, 

 the drier the situation a plant habitually frequents 

 the fleshier are its leaves, and the greater its tend- 

 ency to make the stem share in the work of feed- 

 ing, or even to get rid of foliage altogether. In 

 Australia, however, most of the forest trees, like 

 the eucalyptuses, have got over the same difficulty 

 in a different way ; they arrange their leaves on 

 the stem so as to stand vertically to the sun's 

 rays, instead of horizontally, which saves evapo- 

 ration, and makes the woodland almost entirely 

 shadeless. Many of these Australian trees, how- 

 ever, have no true leaves, but use in their place 

 flattened green branches. 



