40 THE MORPHOLOGY OF PTERIDOPHYTES 



The internal anatomy of the rhizomes varies considerably, 

 according to their size, for those with a diameter of less 

 than I mm are composed of almost pure parenchyma, while 

 large ones possess a well-developed stele. Fig. 6C is a 

 diagrammatic representation of a large rhizome, as seen in 

 transverse section. In the centre is a solid rod of tracheids 

 with scalariform thickenings. As there is no clear distinction 

 between metaxylem and protoxylem, it is impossible to 

 decide whether the stele is exarch, mesarch or endarch. 

 Around this is a region which is usually designated as 

 phloem, although it is decidedly unlike the phloem of more 

 advanced plants, for its elongated angular cells are often 

 lignified in the corners. Surrounding this is a region of 

 'pericycle', composed of elongated parenchymatous cells, 

 and then comes an endodermis with conspicuous Casparian 

 strips in the radial walls. Three zones may often be dis- 

 tinguished in the cortex, the innermost of which is fre- 

 quently dark brown in colour because of abundant deposits 

 of phlobaphene (a substance formed from tannins by oxida- 

 tion and condensation). The middle cortex consists of 

 parenchymatous cells with abundant starch grains, while 

 the outer cortex contains, in addition, the hyphae of the 

 mycorrhizal fungus. In some cells the mycelium is actively 

 growing while in others it forms amorphous partially 

 digested masses. 



In the colourless, or brown, transitional region at the base 

 of the aerial axes, the xylem increases in amount, becomes 

 medullated and spHts up into a variable number of separate 

 strands. This process of medullation continues higher up 

 the stem, as shown in Fig. 6D, and the central pith region 

 becomes replaced by thick-walled fibres. There is here a 

 transition from the protoxylem, with its helical or annular 

 helical thickenings, to scalariform metaxylem tracheids, the 

 protoxylem being exarch. The xylem is surrounded by a 

 region of thin-walled cells, not clearly separable as phloem 

 and pericycle, and the whole stele is enclosed in a well 



