182 GENERAL BOTANY 



The thorns may represent axillary branches, as they 

 are in many other cases. But the barbed hairs ap- 

 pear to be a special structure peculiar to the family 

 to which the Prickly-pear and Cactus belong. The 

 spines, undoubtedly (as in the other cases cited) keep 

 animals from eating the plant, for were it not for them, 

 this luscious watery plant would .soon be eaten by 

 animals, during the hot dry months. But we cannot say 

 that they have been developed with the purpose of keep- 

 ing cattle from eating the shoot. Indeed all these 

 spines and thorns that we have examined, are probably 

 the direct result of the dryness of the air, as was said 

 in the case of ZIZYPHUS and the Barberry. 



ACACIA MELANOXYLON, R. Br. 



The Black-wattle which is planted on hill stations 

 and also occasionally on the plains. 



This tree has alternate lanceolate or elliptic, slightly 

 falcate leaves which stand stiffly upright. 



There are three things about these leaves which at 

 . once strike a careful observer. 



In the first place, they are utterly different from those 

 of all other Acacias. In the White-wattle, ACACIA 

 DEALBATA, for instance, the leaves are bipinnate with 

 numerous small leaflets, and the same is true of all 

 Indian Acacias for example ACACIA ARABICA (fig. 43). 



In the second place, these leaves have no marked 

 midrib, but several veins which start from the base 

 and curving out come together again at the tip a 

 basal venation which is rare among dicotyledons. 



In the third place, one sees at once that if the stalk be 

 not twisted, the plane of the leaf is not at right angles to- 



