PTERIDOPHYTES 



171 



Salviniaccae 



These are the floating ferns, comprising two genera, Salvinia and 

 Azolla, with few species, hut some of them are widely distrihuted. 



Sporophyte. — The sporophyte 

 body is a floating, dorsi ventral stem, 

 that develops hy an apical cell with 

 two cutting faces, instead of the apical 

 cell with three cutting faces usual 

 among ferns. The segments are cut 

 off right and left, and subsequent 

 divisions result in eight rows of cells, 

 four ventral and four dorsal. In 

 Salvinia (fig. 396) the dorsal rows of 

 cells give rise to four rows of broad, 

 flat, overlapping, aerial leaves; while 

 the two central ventral rows give rise 

 to submerged and much dissected 

 leaves that bear the sporangia. The 

 two lateral ventral rows give rise to 

 the branches, and roots are entirely 

 lacking. In Azolla the two central 

 dorsal rows of cells do not give 

 rise to lateral members, but the . Fxo 3 9 6.- Salvinia shomng the 



broad floating leaves, and the dissected 

 two lateral dorsal rows produce submerged leaves bearing sporocarps. 



leaves which are dorsi veri trail y lobed 



(fig. 397). The submerged 

 ventral lobes bear the spo- 

 rangia, and a chamber in the 

 aerial dorsal lobe is inhab- 

 ited by an endophytic alga 

 (Anabaena). The two central 

 ventral rows produce roots, 

 and the two lateral ventral 

 rows produce branches. 



Sporocarp. — The sporangia 

 are submerged, as described, 

 and each sorus is completely 

 invested by the indusium, 



Figs. 397, 308. — Azolla: 397, ventral surface 

 of branch, showing leaves and sporocarps; 

 398, megasporocarp and microsporocarp. — After 

 Campbell. 



