440 SALVINIA LESS. 



Springing from the bases of the submerged leaves are 

 numerous globular capsules (so), each containing a number 

 of sporangia. The wall of the capsule (Fig. 115, A) corre- 

 sponds with the indusium of a fern, and the contained group 

 of sporangia with a sorus. But the sori of Salvinia, unlike 

 those of ordinary ferns, are dimorphic, some containing a 

 comparatively small number of large sporangia (mg. spg\ 

 others a much larger number of small ones (mi. spg). The 

 larger kind, distinguished as mcgasporangia, contain each a 

 single large spore, or megaspore : the smaller kind, or micro- 

 sporangia, contain a large number of minute spores, like 

 those of an ordinary fern, and called microspores. It is this 

 striking dimorphism of the sori, sporangia, and spores which 

 forms the chief distinction between Salvinia and its allies 

 and the true ferns. 



When ripe the sporangia become detached and float on 

 the surface of the water. The microspores germinate (B), 

 while still enclosed in their sporangium : each sends out a 

 filament, which protrudes through the wall of the micro- 

 sporangium, its extremity (spy) becoming separated off by a 

 septum and then divided into two cells. The protoplasm 

 of each of these divides into four sperm-mother-cells, and 

 from these spirally-twisted sperms are produced in the usual 

 manner. It is obvious that the two cells in which the 

 sperms are developed represent greatly simplified spermaries : 

 the single proximal cell (prtK) of the filament arising from 

 the microspore, a still more simplified prothallus. Both 

 prothallus and spermaries are vestigial structures ; the pro- 

 thallus is microscopic and unicellular instead of being a 

 solid aggregate of considerable size, as in the two preceding 

 types ; each spermary forms only four sperm-mother-cells, 

 and the total number of sperms is therefore reduced to 

 eight. 



