HISTOLOGY OF THE RHIZOME. 



121 



5. Phloem-2)arenchyma ; ordinary parenchymatous cells filled with 

 starch, scattered here and there among the bast-fibres and sieve-tubes. 



6. Tracheids (scalariform} or ''ladder-cells" ; occupying most of the 

 central part of the bundle. Their structure calls for some remark. They 

 are empty or air-filled fusiform tubes, whose hard, thick walls are in the 

 young tissue sculptured with great regularity into a series of transverse 

 hollows or pits, which finally become actual holes. The walls of the 

 tracheid are therefore continuous at the angles, but along their plane sur- 



p.3. 



fp. b.s 



FIG. 53. Longitudinal section of a nbro-vascular bundle, surrounded by the fun- 

 damental parenchyma, fo.f, bast-fibres; J>.s, bundle-sheath; /.p, fundamental 

 parenchma ; p.p, phloem-parenchyma ; p.s, phloem-sheath ; s.f, sieve-tubes ; t t 

 scalariform tracheids or ladder-cells ; U'.p, wood-parenchyma. 



faces become converted into a series of parallel bars, making a grating of 

 singular beauty. The slits between the bars are not rectangular passages 

 through the wall, but are rather like elongated, flattened funnels, opening 

 outwards. The sides of the funnels are called the borders of the pits; and 

 pits of this sort are called bordered scalariform pits (cf. Fig. 53). 



7. Trachea? or vessels (spiral} ; scattered here and there among the 

 tracheids, and hardly distinguishable from them in cross-section. They 

 are continuous elongated tubes filled with air, and strengthened by a beau- 

 tiful close spiral ridge (sometimes double) which runs round the inner face 

 of the wall (Fig. 52). 



The tracheids and vessels are of great physiological importance, being 

 probably the main channels for the flow of sap. Sap is water holding 

 yarious substances in solution. The water enters by tlie roots, flows prin- 

 cipally through the walls of the vessels and tracheids, and not thron//Ji 

 their cavities, which are filled with air, and is thus conducted through the 

 rhizome and upwards into the leaves. 



8. Wood-parenchyma ; cells like those of the phloem-parenchyma (5) 

 scattered between the vessels and tracheids. 



