APOGAMOUS FERNS. THE GENERAL PHENOMENON 



grown. The other two species are C. Fortunei and C. caryotideum, both previously treated 

 as varieties. Their separation as species does not, as it turns out, seriously affect the 

 issue. I have had wild material of both C. Fortunei and C. caryotideum and botanic garden 

 material of C. falcatum from several sources including Wisconsin. All are cytologically 

 indistinguishable, although no doubt slightly different genetically, and therefore for 

 the present purpose they can be used interchangeably. Illustrations of the various types 

 of fronds will be found in the next chapter. 



The early stages of sporangial development in all Polypodiaceous ferns, whether 

 apogamous or sexual, are identical, and consist of a limited but very precise set of 



f 













f 



\ A' d 



Fig. 164. Section of part o{ a sorus oi Cyrtomiumf alcatum {h.f.) Presl 

 to show various stages of young sporangia, x 250. 



cleavages in what was originally a single superficial cell. After cutting off the stalk, 

 which soon becomes a short filament, the terminal cell undergoes four oblique cleavages 

 which separate the sporangium wall from a central tetrahedral cell (Fig. 1 65 a) . From 

 this central cell (which appears triangular in all types of section) a further set of cleavages 

 parallel to the four sides separates the tapetum from the archesporium (Fig. 165^, 

 centre). The tapetum gives rise to two layers of cells which are nutritive in function 

 (Fig. 16^ b, left), and the archesporium undergoes a sequence of four synchronized 

 mitoses (cf Fig. 166) to give a central mass of cells which, in the sexual species, are 

 ultimately sixteen in number. These sixteen cells then enlarge considerably and begin 

 to round up to become mother cells, each of which will give rise to four spores after 

 undergoing meiosis. During the rounding up which accompanies the early meiotic 

 prophases the mother cells tend to separate somewhat from each other, and the inter- 

 stices between them become filled with protoplasm from the inner tapetal layer which 

 becomes plasmodial. This is visible in many of the photographs of meiosis in section 

 included in previous chapters. The walls of the inner tapetal cells finally disappear, 



162 



