PTERIDOPHYTA 



281 



the xylem occupying the center of the stem. A cross section of the rhi- 

 zome shows a more or less sclerenchj'matous cortex, a continuous and dis- 

 tinct endodermis, a several-layered pericycle, and a narrow l)ut contin- 

 uous band of phloem. The xylem forms a solid central mass consisting of 

 groups of large scalariform tracheids intermixed with parenchyma. 

 Numerous mesarch protoxylem groups are scattered throughout the meta- 



Fig. 23G. Cross section of the central portion of tlie rliizome of Dennstacdtia pundilobula, 

 an amphiphloic siphonostele, X80. In the center of the pith is a group of thick-walled 

 sclerenchyma fibers. 



xylem. Leaf traces are connected directly with the stele, forming no 

 gaps. 



Amphiphloic Siphonostele. This stelar type may be seen in such well- 

 known ferns as Adiantum and Dennstaedtia, as well as in a number of 

 others. Here the vascular tissues form a cylinder enclosing a pith, the 

 xylem being surrounded both externally and internally by a complete 

 zone of phloem (Fig. 236). 



A transverse section of the rhizome of Dennstaedtia punctilohula, the 

 hay-scented fern of the Eastern United States, shows a thick outer cor- 

 tical region composed of dark-colored sclerenchyma and a thin inner 

 parenchymatous region. An outer endodermis delimits the cortex from 

 the outer pericycle, which consists of two or three layers. Next come 

 the outer phloem, the xylem, the inner phloem, the inner pericycle, the 

 inner endodermis, and the pith. Both the outer and inner phloem are 



