184 GENERAL BOTANY 



directly overhead (its hottest time) than if horizon- 

 tal like an ordinary leaf. Morning and evening the 

 leaf faces the sun more, but its rays are then less 

 intense. The object of the plant in doing without 

 leaflets and having instead a broad vertical green 

 petiole appears, then, to be to escape the sun's rays 

 when they are strongest (cf. on buds pp. 96 & 163). 



FIG. 44 

 ACACIA MELANOXYLON, R. Br. 



Young plant showing the change in the form of the leaves. 



