PTERIS RHIZOME 165 



b. Parenchymatous cells, which fall under the general term 

 conjunctive parenchyma. 



4. An elliptical area of xylem occupies the centre of the 

 bundle, it is composed of 



a. Tracheides, which are large and of circular or polygonal 

 outline, with highly refractive lignified walls, and no cell- 

 contents. Towards the poles of the elliptical area tracheides 

 of small size, and circular outline may be seen : these are the 

 pro to xylem. 



b. Conjunctive parenchyma, distributed irregularly among 

 the tracheides. 



Longitudinal sections should also be cut, so as to traverse 

 the vascular bundles of the central series : they may be treated 

 as above directed. Note in these the thin-walled ground- 

 parenchyma, and the brown sclerenchyma j the latter consists 

 of fibrous cells with peculiar crossed pits in their walls. 



a. The bundle-sheath, or endodermis, consists of narrow 

 oblong cells with square ends, and with brown contents. 



b The phloem-sheath, or pericycle, is composed of wider 

 oblong cells, with colourless protoplasic contents : the ends are 

 square or oblique. 



c. The sieve-tubes appear as wide tubes, with pointed ends, 

 and cellulose walls : the surfaces of the walls which separate 

 contiguous sieve-tubes are covered with numerous sieve-plates 

 (best seen in sections treated with chlor-zinc-iodine), to which 

 round, highly refractive granules adhere : the sieve-tubes are in 

 fact essentially similar to those of Nephrodium (p. 163). Note 

 especially the irregular outline of the wall when seen in 

 longitudinal section : this will be best seen in sections which 

 have been cut so as to pass through the phloem in a plane 

 parallel to the flattened side of one of the larger bundles. 



d. The scaiariform tracheides, which are the main con- 

 stituents of the xylem : they are similar in all their main 

 characters to those of Nephrodium (see p.. 163), being elongated 

 and pointed, while the walls aremarked by transversely-extended 

 bordered pits, arranged regularly so as to give the ladder-like 

 appearance ; but they differ from them in one point, for by a 

 careful examination of fine sections it may be ascertained that 



