PTERIDOPHYTES 



123 



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 nlhoceros 

 is selected as illustrat- 

 ing a possible ancestral 

 condition of vascular 

 plants at Ine 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- 

 nous tissue ; (4) the 

 intervening green 

 tissue develops green 

 expansions (small phyte, except that it is branching. 



Fig. 265. — Lycopodium pithyoides: a sporophyte consist- 

 ing of a. branching stem covered with small leaves, each leaf 

 bearing a sporangium; the simplest tj'pe of Lycopodium sporo- 



