DEVELOPMENT OF PLANTS 



309 



compels our admiration. Comparatively few leaves are pro- 

 duced annually from the tip of each stem — in the preceding 

 order usually but one. This appears to be the rule in under- 

 ground stems, since the difficulty and danger of sending the tender 

 young leaves up through the soil is minimized by the develop- 

 ment of one or a few leaves which 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 differentiated into an epidermis, stomata, 



'^^lL 



Fig. 222. Arrangements of the sporangia: A, lobe of leaf of Dryopteris 

 with sporangia grouped in circular sori, 5. B, sorus enlarged, showing the 

 shield-like membrane or indusium, in, covering the sporangia, sp. C, lobe 

 of leaf of Asplenium with elongated sori, s. 



chlorenchyma and vascular bundles as in the higher plants (Fig. 

 223). The stems are more usually prostrate, creeping rhizomes 

 that branch sparingly and so gradually give rise to colonies of 

 ferns. One of the most attractive features of certain tropical 

 districts is the tree ferns with erect stems of palm-like appear- 

 ance which lift their great crowns of leaves 30 to 50 feet in the air. 

 The vascular bundles are of the concentric type (Fig. 221) and 

 usually form a hollow net-work in the stem (Fig. 220, x). Cer- 



