THE PTERIDOPHYTA : FILICALES, THE FERNS 



509 



are perforated, so that they form true vessels, which are of rare occurrence 

 in Cryptogams (Fig. 502). 



The sieve tubes are of a primitive type, their lateral walls being covered with 

 sieve areas (Fig. 503). It has been found that the parenchyma cells in the 

 phloem are sister cells of the sieve tubes, so that, in character if not in form, 

 they represent companion cells. The phloem cells are all derived from the 

 same cells as those w^hich produce the pericycle and endodermis, and the 

 latter tissue is therefore in this case part of the stele instead of being, as is 

 usual, the innermost layer of the cortex. 



Structure of the Petiole 



The petiole shows, in section, a large number of meristeles, arranged 

 apparently at haphazard. In small petioles, however, with a smaller number 

 of meristeles, it is possible to see that their arrangement is that of a much- 

 convoluted horseshoe, with the open side adaxial, that is, towards the upper 

 side of the petiole. The significance of this arrangement is explained later 

 (p. 561). An irregularly T-shaped band of sclerenchyma lies inside the 

 horseshoe. The meristeles at the two ends of the horseshoe are derived 

 from the inner of the two steles of the rhizome (Fig. 504). 



Fig. 504. — Pteridiiim aquiUnnm. Transverse section of a 

 young petiole before the formation of sclerenchyma. 



Structure of the Root 



The roots arise adventitiously from the rhizome and are blackish and 

 wiry in texture. Their anatomy essentially resembles that of the Dryopteris 

 root, but there are two unusually large metaxylem vessels standing side by 

 side in the middle of the xylem plate. They project sideways into the phloem 



