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. 



Gametophjrte. — The gametophyte of Lycopodium is a very char- 

 acteristic structure and suggests very little connection with the gameto- 

 phytes of liverworts. When the spore germinates, 

 there is produced at first a subterranean tuberous 

 body {primary tubercle), which later gives rise to 

 an aerial, lobed, green portion bearing the sex 

 organs. The gametophyte is differentiated, there- 

 fore, into two distinct regions (fig. 275). 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. This may be regarded as the primitive Fig. 275. — Gameio- 

 kind of gametophyte body in Lycopodirun, but ^rpllL'To^ 

 there are two variations that are important. In the differentiation into 

 certain species the aerial region is lacking, leaving tuberous region and 

 the gametophyte simply a subterranean, tuberous "°™' 

 body (fig. 277), which, of course, bears the sex 

 organs. In certain epiphytic species the tuberous region is lacking, 

 the gametophyte being entirely aerial. 



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- 

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

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



