GROUP III. PTERIDOPHYTA : FILICIN^E ; LEPTOSPORANGIAT^E. 393 



is developed from a single superficial cell. The cell grows so as 

 to project more or less : it is then divided into two cells an outer, 

 the mother-cell of the sporangium ; an inner, the stalk-cellby a 

 wall which may be horizontal (most Polypodiaceae) or oblique 

 (as in Hymenophyllaceae, Schizaeaceae). As the mother-cell of the 

 sporangium grows, it undergoes division by the successive forma- 

 tion of three oblique walls, intersecting one another below at an 

 angle of about 60, and reaching above to the wall of the mother- 

 cell ; at this stage the sporangium consists of three latero- basal 

 external cells surrounding the pointed lower end of a tetrahedral 

 cell, the spherical base of which occupies the summit of the spor- 

 angium. A wall is now formed in the tetrahedral cell, parallel to 

 its spherical free surface, and intersecting the three oblique walls ; 

 so that the sporangium now consists of four peripheral cells, form- 

 ing the wall, and a central cell. From the central cell are cut off, 

 by successive walls parallel to its sides, four cells which give rise 

 to the tapetum by subsequent growth and radial, and sometimes tan- 

 gential, division; the remaining internal tetrahedral cell constitutes 

 the unicellular archesporium from which the spores are derived. 



As the young sporangium grows, it gradually assumes its 

 definitive form (oval-lenticular, as in Polypodiaceae, Cyatheaceae ; 

 discoid, as in Hymenophyllaceae ; ovoid, as in most Schizaeaceae ; 

 globose, as in Ceratopteris and Mohria). The four primary peri- 

 pheral cells undergo repeated radial division, and form the wall 

 of the sporangium, which ultimately consists of a single layer of 

 cells with cuticularised walls : a portion of the wall is in all cases 

 developed to form the ring or annulus, by means of which the 

 dehiscence of the sporangium is effected, the walls of which are 

 specially thickened and cuticularised, coloured yellow or brown, 

 and are elastic. The form and position of the annulus varies in 

 the different groups : in the Polypodiaceae (Fig. 260), where the 

 sporangium is attached to' the stalk by the margin, the incomplete 

 annulus is a projecting row of cells with their longer axes trans- 

 verse, extending round the margin in the plane of the stalk, with 

 which it is connected on one side, but not quite reaching it on the 

 other ; in the Cyatheaceae, in which the form and attachment of the 

 sporangium is similar, the annulus is slightly inclined to the plane 

 of the stalk, and it is quite complete; in the Hymenophyllaceae 

 and Gleicheniaceae, where the discoid sporangium is attached by 

 its under surface, the annulus forms a complete ridge round the 

 margin, more or less nearly at right angles to the plane of attach- 



