132 



THE BIOLOGY OF A PLANT. 



from it the central cell or archesporium. In the parietal cells further 

 divisions follow, perpendicular to the surface, while the archesporium gives 

 rise to four intermediate or tapetal cells, parallel to the original parietal 

 group (Fig. 65, g). The sporangium now consists of a central tetrahedral 

 archesporium bounded by four tapetal cells, which in turn are enclosed 'by 

 the parietal cells, at this time rapidly multiplying by divisions perpen- 

 dicular to the exterior. Owing to the peculiar position of the planes of 



A. 



B. 



O.I. 



FIG. 64. (From Luerssen, after Burck.) Indusia and receptacle of Pteris aquilina ; 

 B (diagrammatic), seen from below ; A^ in the section of the edge of a leaflet, o.i, 

 outer (false) indusium ; j.j, inner (true) indusium ; r, receptacle ; s, young 

 sporangia. 



division the whole capsule is now somewhat flattened, and it becomes still 

 more so by the formation along the edge of a peculiar structure called the 

 ring or annulus, whose function is the rupturing of the capsule and the 

 liberation of the spores. The annulus is formed by a number of parallel 

 transverse partitions (Fig. 65, /, 7*, i,j), which subdivide the peripheral 

 cells of one edge of the capsule until a certain number of cells have been 

 formed. These then project upon the capsule (Fig. 65, j) and form an in 

 complete ring (Fig. 65, k). 



Meanwhile the tapetal cells sometimes subdivide so as to form a double 

 row (Fig. 65, 7i), and soon afterwards are absorbed, space being thus left 



