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 Lycopodium 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 FIG. 275. Gameto- 
becomes reduced, is no longer aerial, and therefore ^^^^S^ 
not green (fig. 275); while in still others it is not de- t he differentiation into 
veloped at all (fig. 277), the gametophyte being sim- tuberous region and 
, , , . ,, crown. After BRUCH- 
ply a subterranean tuberous body bearing the sex 
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 
