PTERIDOPHYTA 



303 



gium wall is only one layer of cells thick and is without an anniilus. The 

 vernation is circinate (Marsileaceae) or folded (Salviniaceae). The 

 gametophytes are greatly reduced, developing largely within the spore 

 wall. The development of the embryo is essentially the same as in the 

 Filicales. The Hydropteridales are a specialized aquatic group, the two 

 families apparently having been derived independently from the Filicales. 



B ^^^^^^^ C 



Fig. 257. Azolla fiUculoides. A, massula with glochidia and enclosed microspores, X250; 

 B, germinating microspore, the antheridium initial dividing, X560; C, male gametophyte 

 with small prothallial cell, large nutritive cell, and external antheridium, X560; D, female 

 gametophjte, showing the large nutritive cell and the extruded tissue bearing archegonia, 

 X65. (After Campbell.) 



COMPARISON OF THE CLASSES OF PTERIDOPHYTES 



The most important distinguishing characters of the four classes of 

 pteridophytes are as follows: 



Psilophytinae. Leaves small, simple, spiral, generally without veins, 

 often wanting. Roots absent. Stem mostly an exarch protostele. Leaf 

 gaps absent. Definite strobili not organized. Sporangia solitary, ter- 

 minal; on elongated branches and unilocular or (in Psilotales) on greatly 

 reduced branches and bilocular or trilocular; tapetum wanting. Homos- 

 porous. Prothallia (in Psilotales) tuberous, subterranean, not green. 

 Sperms multiciliate. Embryo without a suspensor. 



Lycopodiinae. Leaves simple, usually small and numerous, generally 

 spiral, with a single vein. Stem mostly an exarch protostele. Leaf gaps 



