DIV. i PTERIDOPHYTA 499 



leaves also correspond in structure with those of the Phanerogams. 

 The three primary organs in most Pteridophyta grow by means of 

 apical cells (Figs. 100, 101, 156). Such apical cells are not to be 

 recognised in Lycopodium and Isoetes, while Selaginella shows both 

 growth by an apical cell and the transition to growth by a number 

 of initial cells. Stems, leaves, and roots are traversed by well- 

 differentiated vascular bundles, and the Pteridophytes are, in conse- 

 quence, designated Vascular Cryptogams. The bundles of the great 

 majority of Pteridophytes are as a rule constructed on the concentric 

 and radial types (cf. pp. 99 ff., Figs. 463, 464). Secondary growth in 

 thickness, resulting from the activity of a special cambium, occurs only 



FIG. 464. Transverse section of stem of Lycopodium complanatum. ep, Epidermis ; re, li, pp, outer, 

 inner, and innermost parts of the primary cortex, surrounding the central cylinder composed 

 of xvlem and phloem regions ; sc, scalariform tracheides ; sp, annular and spiral tracheides ; 

 v, phloem, (x 26. After STRASBURGER.) 



occasionally in existing forms, but it was characteristic of the stems of 

 certain extinct groups of Pteridophytes. 



The course of the vascular bundles in the leaves (venation) provides important 

 characters for classification, especially in the Ferns (Fig. 465). While only a single 

 median nerve is present in the simple leaves of the Horse-tails and Club-mosses 

 the nerves of the leaves of Ferns branch in the most various fashion ; they may 

 be dichotomous or pinnate and either end freely or anastomose to form a system 

 of meshes. In these polygonal meshes the ultimate branches may end blindly. 



The SPORES are produced in special receptacles termed SPORANGIA 

 (Fig. 466), which occur on the asexual generation, either on the 

 leaves, or less frequently on the stems in the axils of the leaves. 

 The leaves which bear the sporangia are termed SPOROPHYLLS. The 



