PTERIS RHIZOME 297 



IX. Cut longitudinal sections so as to pass tangen- 

 tially through the central network of bundles : treat 

 some sections with chlor-zinc-iodine, and mount others 

 in glycerine. Note the several tissues observed in the 

 transverse sections, and they will show here the same 

 position relatively to one another. By reason of the 

 frequent splittings and fusions of this bundle-system the 

 several elements will appear contorted and twisted, but 

 this does not materially affect their general arrange- 

 ment, which will be seen under a high power as 

 follows : 



A. In the xylem of the bundle 



a. The scalariform tracheides, which are the 

 main constituents of the xylem : they are 

 elongated, prosenchymatous elements, with 

 ladder-like marking of the lateral walls : this 

 is due to the presence of regularly arranged, 

 transversely elongated, bordered pits. (Com- 

 pare those of the pine, p. 240.) Take 

 especial notice of the appearance of the lateral 

 walls as seen in longitudinal section, where 

 two tracheides are contiguous with one another, 

 and compare them with parts of the wall 

 which adjoin. 



I. Cells of the conjunctive parenchyma inter- 

 spersed among the tracheides. 



c. Tracheides with spiral marking : these are the 

 first-formed wood, or protoxylem. 



B. In the phloem observe 



a. The sieve-tubes, which are also elongated ele- 

 ments with pointed ends ; the surfaces of the walls 

 which separate contiguous sieve-tubes are covered with 



