368 SALVINIA LESS. 



As in the fern, the Equisetum plant, reproducing as it 

 does by asexual spores, is the agamobium, the gamobium 

 being represented by the prothallus. The peculiarity in the 

 present case is that the gamobium is sexually dimorphic, 

 some prothalli producing only male, others only female 

 gonads. 



SALVINIA 



Salvinia is a fresh-water plant, consisting of a long floating 

 stem bearing at intervals whorls of leaves. Of these some 

 have the ordinary character while others hang downwards 

 into the water and have the form and function of roots. 

 True roots are absent. 



The sori or groups of sporangia (Fig. 85, A) are borne on 

 the proximal ends of the submerged leaves, each being en- 

 closed in a globular case corresponding to the indusium of 

 ordinary ferns. They differ from the sori of the typical 

 ferns in being 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 megasporangia, contain each a single large 

 spore, or megaspore : the smaller kind, or microsporangia, 

 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. 



The microspore germinates (B), while still enclosed in its 

 sporangium, by sending out a filament, the end of which (spy] 

 becomes 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 



