THE LEAVES. 



133 



(A, fig. 158). In transverse section the vascular bundles 

 are variously placed, being irregularly scattered, or disposed 

 in one or several groups. The single group is most common, 

 with the paired bundles placed so as to form a crescent, or 

 even a complete ring, which is flattened above or triangular. 

 The largest pair is generally median and dorsal (fig. 158), 

 with smaller ones right and left. 



165. (b) The blade. — In broad leaves, the epidermis of 

 the blade is made up of tabular cells, often with wavy lateral 

 walls (fig. 159). In narrow leaves the epidermal cells are 



Fig. 159. — Surface view of epidermis from under side of leaf of bracken 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. 



longer than wide (fig. 160). Over the veins the cells are 

 elongated parallel with the vein. The epidermal cells are 

 generally free from chloroplasts. The epidermis usually 

 consists of one layer, but in some plants becomes several- 

 layered, either to serve as additional protection against eva- 

 poration or for use as a water-storing tissue. (See ^[441.) 



