524 



A TEXTBOOK OF THEORETICAL BOTANY 



one another and must therefore be reckoned as vessels, not simple 

 tracheids (cf. Pteridium, p. 509). 



Leaf trace 



Leaf gap 



Anatomy of the Petiole 



When an entire stem of Osmunda is cut across transversely it will be seen 

 that the actual stem tissue occupies only about one-seventh or less of the 



entire section, and the rest is made up of a 

 mass of closely packed leaf bases, arranged 

 1 M ^B t*3 '^ ^ spiral sequence. Each leaf base con- 



r* « ' wk M tains a petiolar stele surrounded by a dark- 



I I IS H brown band of sclerenchyma, outside which 



is a broad parenchymatous cortex, lozenge- 

 shaped in section. The soft outer tissues 

 are so closely packed together around the 

 stem that their limits are difficult to dis- 

 tinguish in the mass, and they give the 

 appearance of a continuous tissue, which 

 might be mistaken for an extension of the 

 stem cortex. 



Close to the base of each petiole two 

 adventitious roots are given off, which wind 

 their way outwards between the leaf bases 

 and on emerging form a tangle around the 

 outside. The stele in each petiolar trace 

 passes downwards and inwards through 

 the true cortex of the stem and enters the 

 stele as a horseshoe shaped mass of 

 phloem and xylem, the latter having one 

 protoxylem on its inner concave side. It 

 passes into the ring and takes its place there at the base of a gap. Below 

 the point of junction of leaf trace and stele two of the stelar bundles fuse 

 to form the trace, and this is the origin of its curved outline. The xylem 

 of the trace is composed, unlike the stem xylem, of scalariform tracheids, 

 and is surrounded by a zone of phloem and an endodermis. This is sur- 

 rounded by a parenchymatous inner cortex, followed in turn by a sclerenchy- 

 matous band and the parenchymatous outer cortex already referred to. 

 Close to the actual stem the sclerenchyma of the separate petioles becomes 

 fused into a solid mass, which is of great hardness and very difficult to cut. 



Fig. 521. — Osmunda regalis. — Portion 

 of the stele dissected out to show 

 dictyostelic structure. {After 

 Kidston and Givynne Vaiighan.) 



Anatomy of the Root 



The cortex is very broad relative to the stele, and consists of cells which 

 are thin-walled near the endodermis and become progressively thicker-walled 

 outwards (Fig. 522). The stele is protostelic, with a diarch or triarch xylem, 

 separated from the phloem by several layers of parenchyma, and surrounded 

 by a two-rowed pericycle and an endodermis. 



