6o 



BOTANY 



shape as well as the arrangement of the leaves. Usually 

 leaves consist of three regions, a flattened part or blade, 

 a leaf-stalk or petiole, and a leaf base by which it is 

 attached to the stem. If we regard it as an outgrowth 

 from the stem, we find that it assumes 

 its flattened form by developing a wing 

 on each side, the outgrowth itself also 

 becoming flat. If the outgrowth 

 branches and only its branches develop 

 wings we have what is commonly 

 termed a compound leaf (Fig. 23). 



The leaf-stalk is the lower part of the 

 axis of the leaf and it is continued 

 forwards to the tip, the part which has 

 become winged being called the mid-rib. 

 In some cases the whole of the axis of 

 the leaf becomes winged. The leaf is 

 said then to be sessile or to have no 



stalk> M ^ bage of the kaf are yery 



frequently two small outgrowths, of the 

 nature of leaf branches. These are known as stipules. 

 They vary a good deal in shape and size. 



The object aimed at in the distribution of leaves on a 

 tree is the covering of the framework of its head as com- 

 pletely as possible by a thin curtain of leaves, as free 

 from unoccupied gaps as possible ; the leaves themselves 

 must be so arranged that little shading of one part by 

 another shall occur. If we stand under a tree and look 

 up through its branches we find the leaves are not dis- 

 tributed all about the interior of the space occupied by 

 the boughs and branches ; they are seen to be a more or 

 less complete covering to the head. In a humbler type, 

 such as a thistle or a sunflower, the leaves overlap very 

 little, so that practically the whole leaf-surface is ex- 

 posed to the light during at any rate some part of the day. 



The leaves are arranged in various ways upon the 



FlG ' ^Compound 



