134 



MANUAL OF BOTANY 



first ones cut off from the latter four protective cells which 

 form the tajjetum, an investing nutritive layer, which by subse- 

 quent divisions of its cells becomes multicellular. The central 

 cell so formed is the ar cheap or ium. 



Changes now take place in both inner and outer parts of the 

 growing organ. The four peripheral cells undergo repeated 

 division by anticlinal walls, so that the outer coating of the 

 sporangium becomes multicellular, remaining one cell thick. A 

 special band of cells forming part of this wall, running in some 

 cases longitudinally and in some obliquely or transversely, becomes 

 peculiarly' thickened, as in fig- 892, and constitutes the annulus. 



Fig. 892. 



Fig. 892. Section of sorus of Fern, consistingr of a number of sporangiahorne 

 upon a placental outgrowth from the under side of the leaf and covered 

 by an indusium. Each sporangium has running nearly round it a row of 

 thick-walled cells, the annulus. After Kny. 



Its walls are strongly cuticularised, more so than the walls of the 

 rest of the cells, which, however, undergo cuticularisation to some 

 extent. The annulus in most cases does not extend completelj^ 

 round the sporangium, but leaves a few thin-walled cells between 

 itself and the stalls, which cells form the stomium, where 

 eventually the sporangium opens. The tapetum becomes multi- 

 cellular and for a time invests the archesporium, its cells being 

 filled with very granular protoplasm. The archesporium divides 

 repeatedly till it consists of sixteen cells, which are the mother 

 cells of the spores. AVhen this stage is reached, the tapetal cells 

 become disorganised and give rise to a mucilaginous fluid in 



