THE LEAF 39 



to build up with it and the mineral substances brought 

 up from the ground, those peculiar complex bodies, 

 carbohydrates and proteids, which are the plant's real 

 food. Since the action of the leaves, depending as 

 it does on light, is a surface one, nearly all leaves are 

 comparatively broad and thin, or at least a part of. 

 the leaf is so, and this part is called the blade. 



Some leaves, like those of the Sunn-hemp and Poppy, 

 consist of nothing else, the broad blade standing out 

 from the shoot-axis without any intervening stalk ; 

 such leaves are termed sessile. 



But -in most cases the blade is held out away from the 

 axis by a cylindrical stalk, as in the Mango and Cotton, 

 this stalk being called the petiole. In contradistinction 

 to sessile leaves, these are said to be petioled. 



The end of the petiole where it is attached to the 

 axis is often swollen or broadened out, and is called 

 the leaf-base. In some plants the leaf-base is very 

 conspicuous and wraps round the axis. This occurs 

 for instance in the Canna and the Ginger plant. In 

 all grasses and plants like them, e.g. the cereal crop 

 plants and Bamboo, there is no petiole, but only a 

 blade, and a sheathing leaf-base which may be as 

 long as the blade ; and except for a slit down the 

 further side completely encircles the stem like a tube. 

 Notice that in these plants there is a fringe of hairs 

 or of thin tissue passing across the leaf at the line 

 where the blade and sheath join. 



In others again the base of the petiole is distinctly 

 thickened, and is flexible so that the whole leaf may 

 move up and down. It is then called a pulvinus. We 

 shall refer to this again later. A similar swelling 



