PTERIDOPHYTES 



127 



and the sporogenous tissue inside. When the tapetal layer has become 

 a complete investment, the sporogenous cells cease dividing by ordinary 

 division and become mother cells, each of which forms a tetrad of spores 

 by two successive divisions, known as the reduction divisions (see 

 p. 61). 



Beneath the sporangium, which in section is often somewhat kidney- 

 shaped, there is developed a cushion of sterile cells or even a short stalk, 

 known as the subarchesporial pad (figs. 271, 272, 274), in which the 

 vascular elements end. In certain extinct lycopods with very large 

 sporangia, sterile strands or plates radiate from this subarchesporial 

 pad into the large mass of sporogenous tissue, probably being sterilized 

 sporogenous cells. These sterile strands are important to note, as 

 indicating a tendency to divide a large sporangium into chambers. 



Gametophyte. The gametophyte of Lyco podium is a very char- 

 acteristic structure and' suggests very little connection with the gameto- 

 phytes of liverworts. In what is regarded as the 

 primitive form the spore produces at first a subter- 

 ranean tuberous body (primary tubercle), which 

 later gives rise to an aerial, lobed, green portion 

 (crown) bearing the sex organs. The gametophyte 

 is differentiated, therefore, into two distinct regions. 

 The subterranean tuberous part is variable in form 

 and is often highly differentiated into tissue regions 

 (fig. 276); it always contains a characteristic endo- 

 phytic fungus, which inhabits a definite region of 

 the body. In certain species, however, the crown 

 becomes reduced, is no longer aerial, and therefore 

 not green (fig. 275) ; while in still others it is not de- the differentiation into 



velopedat all (fig. 2 7 7), the gametophyte being sim- tuberous region and 

 . . . . . , , crown. After BRUCH- 



ply a subterranean tuberous body beanng the sex MANN> 



organs. In certain epiphytic species the tuberous 



region is lacking, the gametophyte being entirely aerial, but not always 



green. 



Antheridium. The antheridium (figs. 278-282) begins as a super- 

 ficial cell, which enlarges and then divides by a periclinal wall. The 

 outer cell following this division is the primary wall cell, which forms 

 an outer wall of one layer of cells; the inner cell is the primary sperma- 

 togenous cell, which produces a large number of spermatogenous cells, 

 those of the last division being sperm mother cells. This method of 



