282 PLANT MORPHOLOGY 



made up almost entirely of sieve tubes but are separated from the xylem 

 by a small amount of parenchyma. The xylem forms a narrow cyhnder 

 composed of scalarif orm tracheids. Apparently no protoxylem is present. 

 The inner pericycle usually comprises only one or two layers. The outer 

 portion of the pith consists of parenchyma, the central part of scleren- 

 chyma. The continuity of the vascular cylinder is interrupted by the 



Fig. 237. Cross section of the central portion of the rhizome of Osmunda cinnamomea, an 

 ectophloic siphonostele, X 19. 



departure of leaf traces and the gap formed by each is closed above before 

 the next trace is given off. The endodermis is continuous around the 

 margins of the leaf gaps. Because the internodes are shorter in the rhi- 

 zome of Adiantum, the leaf gaps are more numerous. Furthermore, the 

 woody cylinder is wider and consists of both tracheids and parenchyma. 

 Ectophloic Siphonostele. The ectophloic siphonostele differs from the 

 amphiphloic in lacking internal phloem. Although found in Schizaea, it 

 can be seen to better advantage in Osmunda. Here the rhizome is 

 covered with persistent overlapping leaf bases. The outer cortex 

 is extensive and consists mainly of dark-colored sclerenchyma, while the 

 inner cortex is narrow and parenchymatous. The endodermis is distinct 

 and continuous, even where the stele is interrupted by the outward pas- 



