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MANUAL OF BOTANY 



first ones out off from the latter four protective cells which 

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

 quent divisions of its cells becomes multicellular. The central 

 cell so formed is the archesporiuin. 



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

 growing organ. The four peripheral cells undergo repeated 

 division by anticHnal 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, nmning in some 

 cases longitudinally and in some obliquely or transversely, becomes 

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



Fia. 892. 



Fiff. 892. Section of soilis of Fern, consisting of a number of sporangia borne 

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

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

 thick-waUed cells, the annuhis. 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 completely 

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

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

 eventually the sporangium opens. The tapetmn becomes multi- 

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

 filled with very granular protoplasm. The arohesporium divides 

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

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

 become disorganised and give rise to a mucilaginous fluid in 



