DEVELOPMENT OF PLANTS 



323 



another feature about these leaves that compels our admiration. 

 Comparatively few leaves are produced annually from the tip 

 of each stem — in the preceding order usually but one. This 



Fig. 220. Stem in early spring freed from all of its leaves save the young 

 ones, c, near the tip: r, roots; v, vascular bundles in base of petiole; x, region 

 from which the cortex has been removed to show the vascular bundles from 

 the leaves uniting to form a lattice work. 



appears to be the rule in underground stems, since the difficulty 

 and danger of sending the tender young leaves up through the 

 soil is minimized by the development of one or a few leaves which 



Fig. 221. Cross-section of fern stem: v, a concentric vascular strand, the 

 large cells of the xylem being surrounded by the phloem. Each strand is 

 surrounded by a compact layer of cells, the endodermis; si, stereome; cr, 

 cortical region. 



form large blades after reaching the air. In the two remaining 

 orders of Pteridophyta, branches of the underground stems rise 

 above the ground and these bear numerous small leaves, also a 

 rule for stems of this kind. The tissues of the leaves are differ- 



