374 PART III. THE CLASSIFICATION OF PLANTS. 



The growtTi in length of root, stem, and leaf, is effected by an 

 apical growing-point: the growing-point has generally a single 

 apical cell in the Filicinse and Equisetinae (except root and stem 

 of Marattiaceae and Isoetes) ; in the Lyeopodinae (as also in the 

 foregoing exceptional Filiciuae) there is generally a group of 

 initial cells. 



The anatomy of the stem presents considerable variety. The 

 primary stem is in all cases monostelic (p. 152) : it may continue 

 to be monostelic (e.g. Lycopodiaceae, Isoetes, Osmundaceae, etc.), 

 but more commonly it becomes polystelic (most Filicinae), or schizo- 

 stelic (Equisetinae, Ophioglossaceae). The vascular tissue of the 

 wood consists of lignified spiral (protoxylem) and scalariform 

 tracheides, or less commonly vessels; the bast contains no com- 

 panion-cells. The bundles are generally closed (except Botrychium, 

 Helminthostachys, Isoetes) ; cauline (except Isoetes, Osmunda, 

 Equisetinae, where they are common). The relative arrangement 

 of wood and bast in the stele is generally concentric (see p. 175) in 

 the Filicinae and Selaginellaceae, and radial in the Lycopodiaceae : 

 or the bundles maybe conjoint and collateral as in Ophioglossaceoe, 

 Isoetaceae, Osmundaceae, Equisetinae. The pericycle is some- 

 times absent (e.g. Ophioglossum ; some Leptosporangiate Filicinae, 

 in which it is replaced by a layer of the endodermis). 



The anatomy of the root calls for no special remark, except that 

 in the Equisetinae there is no pericycle, but a two-layered 

 endodermis. 



The reproductive organs are sporangia, generally borne on the 

 leaves (sporophylls) but sometimes directly on the stem (e.g. 

 Selaginella). Each sporophyll may bear many sporangia on its 

 inferior (dorsal) surface, as generally in the Filicinae and Equise- 

 tinae ; or a single sporangium on its upper surface (e.g. Lycopodium, 

 Isoefces), or in its axil (Selaginella). 



When the sporophyll bears many sporangia, they are usually 

 arranged in groups ; each group is termed a sorus, and the more 

 or less well-developed cushion of tissue from which the sporangia 

 spring is termed the placenta. The sorus may be naked ; or it may 

 have a membranous covering, the indusium (e.g. many Filicinae). 



In the Filicinae the sporophylls are not confined to any special 

 portion of the shoot, so as to constitute a flower : but in some 

 cases (e.g. Osmunda, Ophioglossaceae, Marsileaceae) they differ in 

 form and structure from the foliage-leaves. In the Equisetinae 

 the sporophylls are highly specialised, and are grouped into cones 



