GROUP Til. PTERIDOPHYTA : FILICTN^ ; LEPTOSPORANGIAT^. 395 



this takes place at a certain part which, though different in the 

 various forms of sporangia, is always closely connected with the 

 annulus and is termed the stomium (see Fig. 258) ; dehiscence 

 begins by a split between (not through) the cells of the stomium. 

 In the Polypodiaceae the plane of dehiscence is at right angles to 

 the long axis of the sporangium, and the stomium is situated on 

 the margin between the end of the annulus and the stalk ; in the 

 Cyatheaceae, where the stomium is included in the annulus, the 

 plane of dehiscence is as in the Polypodiaceae ; in the Hymeno- 

 phyllaceaa and Gleieheniacese the stomium is included in the annu- 

 lus, and the plane of dehiscence is vertical ; in the Schizaeacea3 the 

 stomium is on the more concave surface, and the dehiscence is 

 lateral longitudinal ; in the Osmundaceae the stomium extends 

 from above the annulus, over the summit of the sporangium, and 

 it is in this vertical plane that dehiscence takes place. 



A striking feature in the general morphology of these plants is 

 the presence on the stem and the bases of the leaf-stalks, espe- 

 cially when young, of numerous scaly hairs (ramenta or palece), 

 which consist usually of a single layer of cells, with more or less 

 thickened brown walls ; they are of various shape, and frequently 

 have marginal glandular hairs secreting tannin or mucilage, the 

 latter generally in the neighbourhood of a growing-point of stem 

 or leaf. Less commonly, glandular hairs are developed on the 

 leaves, as in species of Gymnogramme (Gold and Silver Ferns), in 

 which the under surface of the leaves is covered with a yellowish 

 dust, consisting of minute needles of resinous and waxy substances, 

 secreted by the hairs. Root-hairs occur on subterranean stems 

 and leaf -stalks. 



A primary root is developed, probably in all forms, but in no 

 case does it persist in the adult. In the full-grown plant all the 

 roots are adventitious ; they spring in great numbers from the 

 stem or the leaf-stalks. The roots are small and branched ; 

 the branching is lateral, and the growing-points of the young 

 roots are developed each from a single cell of the bundle-sheath 

 (endodermis), termed a rJiizogenic cell, situated opposite to a 

 xylem-bundle of the central cylinder. In most cases the growing- 

 point of the root has a single pyramidal apical cell (see Fig. 115) 

 with three flat sides and a spherical base directed outwards. But 

 in the Osmundaceae the structure of the growing-point is not 

 constant ; there may be a single apical cell of varying form, or a 

 group of initial cells. 



