PTERIDOPHYTES 
I2 3 
sporophyte is a strobilus. The problem, therefore, is how such a leaf- 
less sporophyte (sporogonium) as occurs among bryophytes could 
become a strobilus 
or rather a stro- 
biloid body. 
An explanation of 
the origin of this body 
has been suggested by 
BOWER'S theory of the 
strobilus. The par- 
tially independent spo- 
rophyte of A nthoceros 
is selected as illustrat- 
ing a possible ancestral 
condition of vascular 
plants at the level of 
bryophytes, and the 
possible successive 
changes are outlined 
as follows: (i) the 
sporogenous tissue 
becomes more and 
more superficial (a 
change begun when 
the sporogenous tissue 
is transferred from 
the endothecium to 
the amphithecium) ; 
(2) the continuous 
sporogenous layer be- 
comes broken into 
separate masses by in- 
tercalated sterile tracts 
(a condition present 
among Anthocerota- 
les) ; (3) the separated 
sporogenous masses 
become more super- 
ficial, resulting in an 
alternation of green 
tissue and sporoge- 
FIG. 265. Ly co podium pithyoides: a sporophyte consist- 
intervening green j ng O f a branching stem covered with small leaves, each leaf 
tissue develops greep bearing a sporangium; the simplest type of Lycopodium sporo- 
expansions (small phyte, except that it is branching. 
