GROUP III. PTERIDOPHYTA : FILICIDE. 3G3 



With the possible exception of the Osinundaceae, each sporangium 

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

 project more or less : it is then divkied-krte-fcwo eells an outer, 

 the mother-cell of the sporangium ;_aa inner, the stalk-cell by a 

 horizontal waH. As the mother-cell of the sporangium grows, it 

 undergoes division by the successive formation of three oblique 

 walls, intersecting one another below at an angle of about 60, and 

 reaching above to the wall of the motherrcell ; 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 sporangium. 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, forming 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 tapctum by subsequent 

 growth and radial, and sometimes tangential, division ; the remain- 

 ing internal tetrahedral cell constitutes the unicellular arche- 

 sporium from which the spores are derived. 



As the young sporangium grows, it gradually assumes its 

 definitive form, which is mo^t commonly ovaL-lenticular. The four 

 primary peripheral 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 : thus in the Polypodiaceae (Fig. 217), where 

 the sporangium is attached to the stalk by the margin, the incom- 

 plete annulus is a projecting row of cells with their longer axes 

 transverse, 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. 



As the development proceeds, the formation of the spores takes 

 place in the interior of the sporangium. The archesporial cell under- 

 goes repeated division, with the result that usually sixteen cells are 

 formed, which are the mother-pells of the spores (Fig. 216). At this 

 stage the tapetal cells undergo disintegration, so that the mass of 

 spore-mother-cells floats freely in the liquid thus produced. Each 

 mother-cell then undergoes division to form four spores. 



