THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



617 



Among the Thallophyta there are many examples of homomorphic alterna- 

 tion (see p. 182), but in the living Bryophyta, Pteridophyta and Spermatophyta 

 heteromorphic alternation is the rule, and with advancing evolution the 



><:>.■ 



>'0J\ 



CONDUCTING 

 STRAND 





D 



FOOT 



PROTHALLUS 



Fig. 628. — Psilotum triqiietriitn . Prothallus and embno. A and B, Portions of a 

 prothalli covered with antheridia. C, Portion of prothallus enlarged, showing 

 antheridia and archegonia. D, A young sporeling showing three apices, con- 

 ducting strands and distribution of the Fungus outlined. The foot is still 

 embedded in the prothallus. E, One of the apices of D showing late origin 

 of the conducting strand. {After Hollotvay.) 



difference between the two generations has steadily increased. In Psilotum, 

 on the other hand, the prothallus almost exactly resembles the sporophytic 

 rhizome in structure. 



