PTERIDOPHYTES 



303 



latticework or meshwork with vascular strands composed of 

 phloem and xylem surrounding the large and frequent leaf gaps. 

 In Fig. 170, ft, the relation of the leaf trace and the leaf gap 

 is shown as it would appear in a transverse section. 



STRUCTURE OF PTEEIS (BRACKEN FERN) 



In the bracken fern (Pteris aquilina) the vascular system is 

 more complicated than that of the maidenhair fern. Instead of 

 a nearly complete vascular cylinder of phloem and xylem, broken 

 by infrequent short 

 leaf gaps, as in the 

 maidenhair, the vas- 

 cular cylinder of 

 Pteris is broken 

 by many long leaf 

 gaps into a set of 

 vascular strands, 

 or bundles, which 

 form a coarse mesh- 

 work, with infre- 

 quent union of the 

 strands composing 

 the mesh. This 

 meshwork, when 

 cut across, forms 

 the outer circle of 

 vascular bundles 



seen in Fig. 172. The two inner bundles, flanked on either side 

 by dark masses of skeletal supporting tissues, are formed by a 

 budding process from the outer bundle cylinder in the early 

 stages of the development of the stem. In Fig. 173, a, the sec- 

 tion of the young stem of Pteris is seen to correspond almost 

 exactly with that of Adiantum. In ft, , and d the budding off 

 of a central bundle and the division of the single bud to form 

 the two central bundles of the adult stem are diagrammatically 

 shown. The origin of the two dark masses of skeletal tissue from 



FIG. 172. Section of the rhizome of Pteris aquilina 



.9, internal skeletal tissue composed of thick-walled fibers ; 



fb, vascular strands of the meshwork comprising the 



vascular cylinder of Pteris 



