THE PTERIDOPHYTA : FILICALES, THE FERNS 



529 



The mature sporangium is more or less spherical, with a thick stalk (Fig. 

 527). The wall is one layer thick and is marked across the top of the 

 sporangium by a double row of narrow cells, the stomium, along which 

 the sporangium dehisces. There is no true annulus, but at one side there 

 is a group of thickened cells which may be supposed to represent it. This 



Fig. 527. — Osmunda regalis. Transverse section 

 of sporangiferous pinnule showing the 

 rachis bearing a cluster of sporangia with 

 thick stalks. 



cap-like annulus has been held to be a reduced remnant of a multiseriate 

 annulus of the Botryopteris type. The spores contain chloroplasts when 

 ripe and germinate immediately. They are unable to survive for long after 

 being shed. 



The Gametophyte 



When the spore germinates the extine breaks along a triradiate ridge, 

 formed by the compression of the spores into tetrads in the sporangium. 

 The spore contents emerge and elongate into a cylindrical cell which divides 

 transversely into two unequal portions. The smaller of these develops into 

 the prothallus, while the larger becomes the first rhizoid, which, unlike those 

 of other Ferns, contains chloroplasts. The prothallial cell first forms a row 

 of cells, of which the terminal cell organizes itself by oblique divisions into a 

 pyramidal apical cell, and proceeds to build up the prothallus. Later on it 

 is replaced by a four-sided cell, and still later by a group of similar cells, as 

 in Dryopteris. The resulting mature prothallus has the same cordate shape 

 as in the Leptosporangiatae (Fig. 528), but is rather larger and more massive 

 and continues its growth for several years, resembling the dark green thallus 

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