PTERIDOPHYTA 



287 



primary sporogenoiis cell (Fig. 243B). The sporangium wall is developed 

 by subsequent anticlinal divisions from the cap cell and the three upper- 

 most stalk cells (Fig. 243C). In all the Filicales the sporangium wall 

 remains one layer of cells thick. 



A unique feature is introduced by the formation of the tapetum from 

 the primary sporogenous cell rather than from the wall tissue. The tape- 

 tum arises from four cells, one of which is cut off each of the four faces of 

 the primary sporogenous cell (Fig. 243Z), E). The tapetum may remain 



Fig. 242. Sporangia of some leptosporangiate ferns. A, B, C, Osmunda regalis, with 

 rudimentary annulus; D and E, Anemia phyllitidis, with apical annulus; F and G, Glcichenia 

 circinata, with equatorial annulus; H and /, Hymenophyllum dilatum, with oblique annulus. 

 (After Wettstein.) 



single-layered, but in nearly all the Filicales its cells divide periclinally 

 to form two layers (Fig. 243F, G). The innermost cell of the sporangium 

 meanwhile undergoes division to form the spore mother cells (Fig. 

 2A3F-H). When these round off, the tapetum disorganizes and forms a 

 Plasmodium that later surrounds them. The number of spores formed in 

 each sporangium exhibits considerable variation, but is relatively high 

 (up to 512) in the lower families and relatively low (commonly 64 or less) 

 in the advanced families. This tendency to reduce the spore output is a 

 significant feature of Filicinean evolution. 



Gametophyte. The typical gametophyte of the Filicales is entirely 

 aerial and consists of a flat, green, heart-shaped thallus usually about 6 

 mm. in diameter (Fig. 244). Numerous unicellular rhizoids grow from 

 its ventral surface into the soil. There is no endophytic fungus, except in 

 the Gleicheniaceae and sometimes in the Schizaeaceae and Hymenophyl- 

 laceae. In Hymenophyllum the gametophyte is an irregularly branched 



