458 



COENOPTERIDEAE 



[CH. 



consists of tracheae, cut across in an oblique direction, which are 

 about to pass out as a separate strand. The centre of the stele 

 is occupied by parenchymatous tissue in which are included 

 scattered tracheae, either singly or in small groups. These 

 medullary tracheae are rather narrower than those of the main 

 xylem cylinder. A characteristic feature is the radial outward 

 extension into the xylem of the medullary parenchyma, which 

 tends partially to divide the tracheal cylinder into broad groups. 



Fio. 315. Ankyropteris corrugata (Will.). Petiolar vascular strand. [From a 

 section in the University College (London) Collection ; after Tansley 

 x35.] 



Fig. 314, A, enlarged from fig. B, a, shows the mesarch 

 position of a protoxylem group, and a few of the parenchyma- 

 tous cells of one of the narrow arms of the axial tissue. At 

 Sv in fig. D a group of large sieve-tubes is seen separated from 

 the xylem by a few parenchymatous cells, and beyond the 

 sieve-tubes are some tangentially elongated cells. Both the 



