438 SALVINIA LESS. 



some (c) remain small and simple, and produce only 

 spermaries (spy) ; others (D) attain a complicated form and 

 a length of over a centimetre, and produce only ovaries 

 (ovy). Thus although there is no difference in the spores, 

 the prothalli produced from them are of two distinct kinds, 

 the smaller being usually exclusively male, the larger female. 



The oosperm develops in much the same way as in ferns : 

 it divides and forms a polyplast, which, by formation of a 

 stem, root, foot, and two cotyledons, becomes a phyllula 

 and grows into the adult plant. 



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 small fresh-water plant, found floating, like 

 duckweed, on the surface of still water. 



The stem (Fig. 114, st) is an elongated slender rhizome 

 floating at or near the surface, and distinctly divided into 

 nodes and internodes. Each node gives off three appen- 

 dages, two broad, flat foliage-leaves (f. /. 1-3 ; f. 1. i' 3'), 

 which lie above the surface of the water, and a branched 

 structure (s. I. 1-3) which has all the appearance of a root, 

 its thread-like branches hanging down into the water and 

 being covered with hairs. The study of their development 

 shows, however, that these organs arise exogenously from 

 the node and have no root-cap : they are, in fact, not roots, 

 but submerged leaves, performing the function of roots. 



