STRUCTURE OF FERN-STEM 



293 



walled cells having comparatively few contents. Next follow one 

 or two layers of much smaller cells, constituting the so-called 

 phloem-parenchyma (P.p.). Immediately within is a conspicuous 

 zone consisting chiefly of large, empty-looking, thin-walled 

 elements, the sieve-tubes (5.), which are often absent towards 

 the ends of the strands, and which are separated from the central 

 xylem by a zone of small-celled wood-parenchyma (X.p.). The 

 bulk of the xylem consists of large tracheids (Xy.), but near the 

 ends and towards 

 the centre of the 

 strand, small ele- 

 ments, representing 

 the protoxylem 

 (P.xy.), can usually 

 be recognised. 

 There is generally a 

 small central mass of 

 wood-parenchyma. 



In longitudinal 

 sections, cut so as 

 to pass radially 

 through a stele 

 (Fig. 161), the sieve- 

 tubes (s.) appear as 

 elongated structures 

 with tapering ends, 

 and bearing the 

 sieve areas on their 

 sloping radial faces. 



E.J.S. 



Scli 



FIG. 161. On the left, part of a radial longi- 

 tudinal section through one of the steles of 

 the Bracken-rhizome, e., endodermis ; g.t., 

 ground tissue ; p., pericycle ; p.p., phloem- 

 parenchyma ; s., sieve-tube ; t., tracheid of 

 the metaxylem ; x.p., xylem-parenchyma. 

 On the right, a little of the sclerenchyma 

 (Scl.) in longitudinal section. 

 Under the high 



power these sieve areas exhibit a rather irregular ladder-like 

 (scalariform) thickening of shining bars, with intervening darker 

 zones exhibiting a fine dotting and bearing a number of highly 

 refractive adhering granules. The longitudinal walls of the 

 metaxylem tracheids (t.) exhibit several rows of closely arranged 

 oblong bordered pits, producing an exceedingly characteristic 

 type of scalariform thickening, whilst the protoxylem tracheids 

 are spirally thickened in the usual way. 



The number of vascular strands observed in a transverse 



