442 SELAGINELLA ikss. 



and embryo, after the manner of the yolk in the eggs of the 

 crayfish and dogfish. 



The protoplasm of the megaspore (c) divides and forms a 

 prothallus {prth) in the form of a three-sided multicellular 

 mass projecting from the spore, which it slightly exceeds in 

 size. Three ovaries {ovy) are formed on it, having much 

 the same structure as in ordinary ferns : if neither of these 

 should be fertilised others are developed subsequently. 

 Thus the reduction of the prothallus produced from the 

 megaspore, although obvious, is far less th;)ti in the case of 

 that arising from the microspore. 



We see that sexual dimorphism has gone a step further in 

 Salvinia than in Equisetum : not only are the prothalli 

 differentiated into male and female, but also the spores from 

 which they arise. 



Impregnation takes place in the usual way, and the 

 oosperm divides to form a polyplast, which, by differentiation 

 of a stem-rudiment, a cotyledon, and a foot, passes into the 

 phyllula stage : no root is developed in Salvinia. By the 

 gradual elongation of the stem (d, st) and the successive 

 formation of whorls of leaves (/), the adult form is assumed. 



Thus the life-history of Salvinia resembles that of the 

 fern, but with two important differences : the spores are 

 dimorphic, and the gamobium, represented by the male and 

 female prothalli, is greatly reduced. 



Selaginella 



Selaginella, one of the club-mosses, is common on hill- 

 sides in many parts of the world. In the commoner species 

 there is a creeping stem which forks repeatedly in the hori- 

 zontal plane, and bears numerous small, close-set leaves, 

 giving the whole plant much the appearance of a moss. 



