140 



PRINCIPLES OF PLANT-TERATOLOGY. 



or archegonium. This is the normal course. It is 

 now known, however, that in a good many ferns, e. g. 

 Pteris cretica (fig. 38), in which the phenomenon was 

 first discovered by Farlow, the prickly shield-fern 

 (Polystichum aa ilea turn) (PI. XI, fig. 2), the hart's- 

 tongue fern (Scolopendriam vulgare) (PI. XI, fig. 4), 

 and Neplirodium pseudo-mas (PI. XI, fig. 5), adventi- 

 tious shoots and roots, developing into young fern- 

 plants, may from time to time, under conditions which 



Fig. 38. — Pteris cretica. Sporophyte growing apogamously.on prothallus. 

 (After De Bary.) %>, prothallns ; w, root of sporophyte. b\ first leaf 

 of sporophyte ; v, second leaf of sporophyte. 



do not as yet seem to be adequately determined, arise 

 from either the upper or lower surface of the pro- 

 thallus ; usually from the latter, when they appear to 

 develop in the same region as, and to replace, or even 

 to grow out of, the archegonia. This phenomenon, 

 known as apogamu, represents clearly, like all other 

 cases of adventive bud-formation, a short cut in the 

 mode of reproduction, and it entirely replaces and 

 supersedes the sexual method. 



Adventitious Prothalli. 



Of great interest is the phenomenon of apospory in 

 ferns. It is also an instance of the formation of 



