PTERIDOPHYTES 



169 



root, while the two inner (dorsally directed) quadrants develop the stem 

 and leaf, which pierce through the tissue of the overlying thallus (fig. 

 394). Recent investigations, however, indicate that both the outer 

 (ventral) quadrants may develop the foot, the two inner-(dorsal) quad- 

 rants forming a tissue that gives rise 

 to leaf, stem, and root. This method 

 of development suggests that of the 

 embryo of Ophioglossales, with which 

 group the Marattiaceae seem to be 

 closely allied. 



Among Polypodiaceae the first divi- 

 sion of the egg is parallel with the 

 long axis of the archegonium (fig. 

 395), the shoot cell being directed 

 towards the apex (notch) of the pro- 

 thallium. At the quadrant stage (fig. 

 395) the two apically directed quad- 



Fics. 394, ,395. — Diagram of quad- 

 rant stage of embryo: 394, Marattia, in 

 which the first wall is transverse to the 



rants give nse to stem and leaf (the io„g ^xis of the archegonium, the 



shaded quadrants giving rise to stem 

 and leaf; 395, a polypod, in which the 

 first wall is parallel with the long axis 

 of the archegonium, the shaded quad- 

 rants (directed towards the notch) giv- 

 ing rise to stem (upper quadrant) and 

 leaf (lower quadrant); in both cases 

 the notch is to the right. 



latter being ventral) ; while the two 



basally directed quadrants give rise 



to foot and root (the latter being 



ventral). The stem and leaf, directed 



towards the notch, grow under the 



prothallium and the leaf turns up 



through the notch. In such a case, 



the foot is the most temporary organ, functioning only so long as the 



gametophyte endures, while the primary root does not exist for a long 



time. The relatively permanent structures of the sporophyte are the 



stem, giving rise to secondary roots, and the leaves. 



Apogamy and apospory. — The phenomenon of apogamy is so preva- 

 lent among Filicineae that it deserves special mention. Apogamy is 

 the production of a sporophyte by a gametophyte without the act of 

 fertilization. Such a sporophyte may arise either from the unfertilized 

 egg, in which case the apogamy is called parthenogenesis ; or it may arise 

 from the vegetative tissue of the prothallium, in which case it is called 

 vegetative apogamy. It is found to be very common among ferns, and 

 can be induced with litde difficulty. 



The companion phenomenon is apospory, in which a gametophyte 

 is produced by a sporophyte without the formation of a spore. These 



