324 



STRUCTURE OF THE STEM 



entiated into an epidermis, stomata, 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 appearance which lift their great 

 crowns of leaves 30 to 50 feet in the air. The vascular bundles 

 are more commonly of the concentric type (Fig. 221). Perhaps 

 we should repeat that a bundle is composed of two types of elon- 

 gated cells, thick walled woody cells (the xylem) and thin walled 



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

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

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

 of leaf of Aspknium with elongated sori, i. 



cellulose cells (the phloem). In the fern type of concentric 

 bundle the phloem surrounds the xylem. These bundles are 

 united in the stem in a variety of ways. In the simplest case 

 we find a central core of xylem surrounded by phloem which 

 gives off strands or vascular bundles to the leaves and roots. In 

 the majority of ferns this core of xylem is modified by the develop- 

 ment of rather delicate pith-like cells in its center, consequently 

 it appears as a ring in cross section or as a cylinder in longitudinal 

 view. This ring is broken into segments owing to the fact that 



