PTERIDOPHYTA 



273 



ever, are not borne in a "fertile spike" but on the abaxial side of the 

 leaves. Generally the fertile and sterile leaves are alike in form, but in 

 Danaea they are different. The sporangia are sessile and borne in distinct 

 sori, these being generally in two rows. In Angiopteris the sporangia are 

 free (Fig. 228.4), but in Marattia and Danaea they are united to form 

 synangia. The synangia of Marattia are superficial, oval, or rounded, and 

 borne near the ends of the veins (Fig. 228B) . The synangia of Danaea are 

 sunken, linear, and borne along the veins; they cover almost completely 

 the backs of the fertile leaflets (Fig. 228C). 



Fig. 229. Section through two sporangia of Angiopteris evecta, showing wall, tapetum, 

 and sporogenous tissue, X200. 



All the sporangia in a sorus originate at the same time, each arising 

 from a single initial cell. The sporogenous tissue is differentiated early 

 and, from the cells immediately surrounding it, the tapetum, consisting 

 of one or two layers, is derived (Fig. 229). The tapetum breaks down 

 when the spore tetrads are formed, its substance being absorbed by the 

 developing spores. It does not form a Plasmodium. The sporangium 

 of the Marattiales is a relatively large structure, producing a great many 

 spores (1 ,500 to 7,000) . A rudimentary annulus is present in Angiopteris, 

 but there is none in Marattia and Danaea. Dehiscence takes place by 

 means of a median slit or, in Danaea, by a terminal pore. 



Gametophyte. The Marattiales have a comparatively large gameto- 

 phyte, sometimes reaching a length of 3 cm., and consisting of a flat, dark 

 green thallus that may be heart-shaped, orbicular, or irregularly^ lobed. 

 It resembles the gametophyte of the Filicales except that it is relatively 

 thick, long-lived, and, as in the Ophioglossales, provided with an endo- 

 phytic fungus. The median portion of the prothallium forms a thick 



