370 SALVINIA LESS. 



instead of being formed of a distinct wall and an internal 

 mass of cells ; and the number of sperms is reduced to 

 eight. 



The contents of the megaspore are divisible into a com- 

 paratively small mass of protoplasm at one end and of starch 

 grains, oil-globules, and proteid bodies, which fill up the rest 

 (c, c} of the spore. The megaspore has in fact attained its 

 large size by the accumulation of great quantities of plastic 

 products, which serve as nutriment to the future prothallus 

 and embryo. 



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

 prothallus (prtJi] in the form of a three-sided multicellular 

 mass projecting from the spore, which it slightly exceeds in 

 size. Several ovaries (pi-y) are formed on it, having much 

 the same structure as in ordinary ferns. Thus the reduction 

 of the prothallus produced from the megaspore, although 

 obvious, is far less than 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. 



