PTERIS RHIZOME 163 



A. In the xylem of the bundle 



a. The scalariform tracheides, which are the main con- 



stituents of the xylem : they are elongated, prosenchy- 

 matous 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. 1 38.) Take especial notice 

 of the appearance of the lateral walls as seen in 

 longitudinal section, where two tracheides are con- 

 tiguous with one another, and compare them with 

 parts of the wall which adjoin. 



b. Cells of the conjunctive parenchyma interspersed 



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 elements with 

 pointed ends ; the surfaces of the walls which separate con- 

 tiguous sieve-tubes are covered with numerous sieve-plates 

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

 round, highly refractive granules adhere : these stain yellow 

 with chlor-zinc-iodine. Note also the irregular beaded appear* 

 ance of the walls when seen in longitudinal section. 



b. Cells of the conjunctive parenchyma, interspered among 

 the sieve-tubes. 



As the vascular bundles of the rhizome of Pteris the (common 

 Bracken) are more regularly typical of the concentric bundle of 

 the Fern than those of Nephroditim, thin transverse sections 

 should also be prepared from the rhizome of this plant ; mount 

 some of the sections in glycerine, others in chlor-zinc-iodine ; 

 or these sections may be stained with haematoxylin and mounted 

 in Canada balsam. 



First examine the whole section under a low power, and note 

 its oval form, the upper and lower surfaces being flattened. It 

 is composed of the following tissues : 



i. A peripheral band of brown sclerenchyma, outside which 

 is a scarcely distinguishable epidermis. 



M 2 



