THE LEAVES. 



109 



(See also \ 93.) The leaves of buds are sometimes thick- 

 ened by the deposit of food material, and when such buds 

 loosen from the plant they may produce a new plant, as in 

 the tiger-lily (see \ 299). Both base and blade may be used 

 for storage, as in the century -plant; or the entire leaf may 

 serve the same purpose, as in the cultivated cabbage. 



136. Structure. — Three regions in each part may be distinguished, as 

 in the root and stem : (i) the epidermis ; (2) the cortex ; both continu- 

 ous with that of the stem ; (3) the steles, continuous with those of the 

 stem when the latter contains several steles, or branches of it when the 

 stem contains a single stele. 



The structure of the petiole agrees in all essentials with that of the 

 stem (see Tf 107, fl. ). The following is a brief summary of the structure 

 of the blade of a foliage leaf. 



Fig. io4.« — Surface view of epidermis from under side of leaf of braclcen fern {Pteris), 

 showing wavy cells, except over veins, v, where they 'are elongated, st, stomata. 

 The dot in each cell represents the nucleus. Highly magnified. — After Sedgwick 

 and Wilson. 



137. Epidermis. — In broad leaves, the epidermis of the blade is made 



up of tabular cells, often with wavy lateral walls (fig. 104), and, except 



in shade plants, usually without green color. It usually consists of one 



layer, but in some plants becomes several-layered, either to serve as ad- 



