GROUP III. PTERIDOPHYTA : FITJCINJE. 365 



a subterranean stolon which eventually throws up at its apex a 

 whorl of green leaves, thus constituting a new plant ; but also 

 frequently from the lamina, as in Asplenium (Diplazium} celtidi- 

 folium. A. bulbiferum, and other species. The bud originates from 

 a single epidermal cell. 



General Histology. The structure of stem, petiole, and root, is 

 characterised throughout by the presence of hypodermal layers, 

 and, generally, of scattered strands of sclerenchymatous tissue, 

 consisting of more or less elongated ground-tissue cells with more 

 or less thickened brown-coloured walls ; and by the predominance 

 of scalariform vascular tissue in the xylem which consists, with 

 but few exceptions, of tracheides. 



The stem is, at its first development, monostelic, with a single 

 axile stele : in some forms this structure obtains (with or without 

 pith) throughout the whole stem (e.g. Hymenophyllaceae, Lygodium, 

 Schizaea) : in the Osmundaceae also the stem is monostelic through- 

 out, the stele eventually consisting of a ring of bundles enclosing 

 a pith : in the other families the stem becomes polystelic. 



In the monostelic stem the bundles are sometimes conjoint and 

 collateral (e.g. Trichomanes among Hymenophyllaceae, Osmun- 

 daceae) : in all other cases the arrangement of the bundles in the 

 stele is concentric, or, more strictly speaking, bicollateral (p. 123), 

 since the phloem does not quite completely surround the xylem- 

 bundles. The concentric steles are cauline and usually consist 

 of two wood- and two bast-bundles, with usually an endodermis 

 and a pericycle: in some cases, however, where the stele is 

 small (e.g. some species of Polypodium) there is no pericycle, 

 its place being taken by a layer of cells formed by the division 

 of the primitive endodermis (p. 115) into two layers. 



In the polystelic stem the course of the steles is such that they 

 form a meshwork, each mesh corresponding to the insertion of a 

 leaf : the bundles of the leaf join those forming the corresponding 

 mesh in the stem. The form of the mesh is determined by the 

 number and insertion of the leaves : when the leaves are numerous 

 and closely arranged, the meshes are relatively short and broad ; 

 when the leaves are few and scattered, the meshes are long and 

 narrow. 



In a monostelic stem, such as that of Osmunda, though the 

 bundles are numerous, no such meshwork is formed. The bundles 

 are here common. A single bundle enters the stem from each leaf, 

 runs straigh-t through several internodes, and then curves to join 



