76 VASCULAR CRYPTOGAMS 



side. Besides a few narrow spiral tracheides, lying at definite points 

 of the transverse section, the xylem consists mainly of scalariform 

 tracheides^ i.e. of tracheides with bordered pits which usually have the 

 appearance of transverse clefts, their ends being mostly obliquely trun- 

 cate or fusiform and pointed. True vessels occur but rarely in ferns ; 

 e.g. in Pteris aquilina and in the rhizome of Athyrium filix-fcemina, 

 where they are also scalariform. Between the tracheides lie narrow 

 thin-walled cells which contain starch in winter. In the phloem, in 

 addition to narrow parenchymatous cells, are sieve-tubes with well- 

 developed sieve-plates, but forming true callus only in a small number 

 of cases ; and at the circumference narrow bast-like thick-wall prosen- 

 chyme. Each individual bundle is usually immediately enclosed in a 

 single distinct layer of narrower cells, the vascular bundle-sheath or 

 endoderm ; this layer, which probably originates from the fundamental 

 tissue, displays a strong tendency for its walls to become brown and 

 suberised. In very young stems and those which permanently remain 

 very slender, as in many Hymenophyllacese, there is a single axial 

 bundle. But in stouter stems and leaf-stalks the central bundle is re- 

 placed by a network of anastomosing bundles, presenting, in typical 

 cases, a cylinder of considerable diameter, by which the fundamental 

 tissue is separated into an outer cortical and an inner medullary portion ; 

 but isolated scattered bundles also arise in addition. The principal 

 bundles which constitute this cylindrical network mostly have the form 

 of broad plates with the margins curved outwards, each surrounded by 

 its thick firm brown sclerenchyme-sheath ; they usually present the 

 appearance of an interrupted ring near the periphery, but in Osmun- 

 daceae the ring is more continuous. From the margins of these cauline 

 bundles spring the more slender filiform bundles which pass into the 

 leaves, the number of openings in the meshes of the cauline 'vascular' 

 cylinder corresponding to that of the leaves. In the leaf-stalk the 

 bundles may either run separately or may coalesce into plates. Ter- 

 letzki (Pringsheim's Jahrb., 1884, p. 452) has detected continuity of 

 protoplasm between the cells in the parenchyme of the rhizome in 

 several species of fern ; the intercellular spaces also contain protoplasm 

 which is in connection with that of the cells. In the aquatic genus 

 Ceratopteris the stem contains large air-cavities. Elongated tannin-sacs 

 occur in the parenchyme of the stem and leaf-stalk of many ferns, 

 especially in the neighbourhood of the ' vascular ' bundles. Gum- and 

 mucilage-cells are also of frequent occurrence. Incrustations of cal- 

 cium carbonate are not infrequent on the leaves. Round stalked glands 

 occur in the fundamental tissue of the stem and leaves of Aspidium 

 filix-mas. 



