t HAP. I. THE LEAF. 51 



SECTION IV. 



The Leaf. 

 THE leaf, which is a temporary part of the plant, Its defini- 



tion. 



is a thin and flat substance of a green colour, issu- 

 ing generally from the extremity of the branches, 

 but sometimes also immediately from the stem or 

 root, and distinguishable by the sight or touch into 

 an upper and under surface, a base and an apex, 

 with a midrib and lateral nerves. But to this defi- 

 nition there are no doubt a good many exceptions. 

 For leaves are not always thin and flat, nor are they 

 always green. The leaves of the Aloe and common 

 House-leek are thick and fleshy, the leaves of the 

 Beet are of a dark and dull purple ; and the leaves 

 of Canary Reed-grass are variegated with stripes of 

 green and white. Nor are all leaves furnished with 

 a midrib and lateral divisions ; for in the Grasses the 

 nerves are parallel. 



The point by which the leaf is attached to the Regions, 

 plant is the base ; the terminating point opposite 

 is the apex ; the intermediate body of the leaf is the 

 expansion, and the boundary of the expansion is 

 the margin. The base is sometimes merely a point 

 in which the expansion originates, forming, as it 

 were, part of the margin, and connecting the ex- 

 pansion immediately with the branch or stem. In 

 this case the leaf is said to be sessile or sitting 

 (PI. II. Fig. J.), as in that of Germander Speed- 



E2 



