320 



THE PTERIDOPHYTES 



sometime fail to develop archegonia or the archegonia do 

 not function, but the sporophyte generation arises as a bud-like 

 outgrowth from the prothallium. In other cases the egg may 

 develop without fertilization (parthenogenesis). Such suppres- 

 sions of sexuality with the development of a succeeding genera- 

 tion asexually are called apogamy. The phenomenon has been 

 noted before in the water molds (Sec. 262) and other fungi, and 

 it is found in various groups throughout the plant kingdom. 



Apospory is the suppression of the process of spore formation 

 and the development of a gametophyte generation directly from 

 the sporophyte. It is found in many of the ferns, which are also 

 apogamous, and is shown by the presence of prothallia, which 

 are direct outgrowths from the fern frond 

 in the place of the sporangia, or sometimes 

 at the tips. Apospory is also found in 

 certain mosses where protonema may 

 develop directly from portions of the stalk 

 and spore cases of the sporophytes. 



Apogamy and apospory are both short 

 cuts in the life histories, which are believed 

 to be due to some unusual life conditions 

 that interfere with the regular develop- 

 ment of gametes and spores in the normal 

 life histories, established during the evolu- 

 tion of plant groups. 



312. The water ferns. The water ferns 

 (order Hydropterales) include four inter- 

 esting genera (Marsilia, Pilularia, Sal- 

 vinia, and Azolla}, each of which is 



/.floatingleaves; r.highly remarkable for some peculiarity of StrUC- 



ture. Salvinia (Fig. 280) and Azolla are 

 floating aquatics, and Marsilia and 

 Pilularia are either aquatic or grow in very wet places. These 

 habits give the common name of water ferns to the group. They 

 are important illustrations of the condition called heterospory, 



FIG. 280. A water fern 

 (Salvinia) 



modified leaf acting as a 

 root; 5, spore fruits. 

 After Pringsheim 



