PTERIDOPHYTA 



279 



phyll, and vascular bundles. As a rule, the mesophyll is uniform. In 

 nearly all the Filicales the root has a stele with two protoxylem groups. 

 The stems of the Filicales display four different stelar types, being 

 either a protostele, an amphiphloic siphonostele, an ectophloic siphono- 

 stele, or a dictyostele. The dictyostele is the most common as well as the 



Fig. 234. Median longitudinal section through the root tip of Pteris gigantea, showing 

 tetrahedral apical cell from which all the other cells have been derived; e, epidermis; c, 

 cortex; s, stele. (After Hof.) 



most advanced type, but the amphiphloic siphonostele is also of rather 

 frequent occurrence. The other two types are uncommon. 



The protoxylem consists of spiral tracheids — elongated cells with spiral 

 thickenings on their walls. The metaxylem is made up almost entirely of 

 scalariform tracheids. These are elongated cells, pointed at each end, 

 with transverse bands of thickening resembling the rungs of a ladder. In 

 nearly all the Filicales the development of the wood is mesarch, the 

 protoxylem being surrounded on all sides by the metaxylem (Fig. 239). 

 Generally the phloem surrounds the xylem, the vascular tissues thus 



