LYCOPODINEAE. LIGULA TAE. 



297 



the stem close beneath the apical cell and above the youngest leaves ; the single leaf- 

 trace-bundles become connected with the cauline bundles only at a later period, as is the 

 case also in the Lycopodiaceae. The vascular bundles of the stem are similar in structure 

 to those of the true Ferns. They are for the most part ribbon-like in form ; a central 

 xylem, composed chiefly of tracheides thickened in a scalariform manner, is entirely 

 surrounded by thin-walled phloem (Figs. 243, 245) ; the primary elements of the xylem, 



FIG. 244- Selaginella inaequalifolia. Longitudinal section through the right side of the axis 01 a spike S ; b the 

 base of the leaf, the ligule, sp the sporangium, V point of union of the bundles of the stem and leaf, / air conducting 

 intercellular spaces with cell-rows ^flying across them. 



very narrow tracheides (Fig. 243), are formed at the extremities of the bundle, and the 

 development and lignification of the broader tracheides proceeds from them to the 

 centre (Fig. 243). The layer of phloem surrounding the xylem is itself surrounded by 

 two or three layers of parenchyma, which Russow compares with the phloem-sheath of 

 the Ferns, and which must at all events be regarded as a bundle-sheath belonging to 

 the fundamental tissue and enclosing the bundle inside the air-space mentioned above. 

 There is no bounding layer with undulated lateral walls (endodermis) in stem or leaves. 

 The vascular bundles in the leaves are slender and of simple construction ; the xylem 

 enclosed in a scanty phloem is composed of spirally or reticulately thickened tracheides. 

 The short and simple stem of the Isoeteae is two- or three-lobed, the lobes being 

 separated from one another by the longitudinal furrows from which the roots proceed. 

 This tuber-like stem has no pith but an axile bundle formed by the union of the inner 

 ends of the leaf-trace-bundles, one of which comes from each leaf. This axile bundle is 

 composed on the transverse section of a roundish polygonal mass of short fusiform 



