ENDOGENOUS PROTHALLUS 367 



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, consists of a long 

 branching stem bearing numerous close-set leaves. It thus 

 resembles in external appearance a moss, but the essential 

 difference between the two is seen from a study of their 

 histology, Selaginella having a distinct epidermis and 

 vascular bundles like the other Vascular Cryptogams. 



The branches terminate in cones (Fig. 87, A) formed of 

 small leaves (sp. pJi) which overlap in something the same 

 way as the scales of a pine-cone. Each of these leaves is a 

 sporophyll, and bears on its upper or distal side, near the 

 base, a globular sporangium. The sporangia are fairly 

 uniform in size, but some are megasporangia (ing. spg) and 

 contain usually four megaspores, others are microsporangia 

 (mi. spg) containing numerous microspores. 



The microspore (B) cannot be said to germinate at all. Its 

 protoplasm divides, forming a small cell (prtli), which repre- 

 sents a vestigial prothallus, and a large cell, the representative 

 of a spermary. The latter (spy) undergoes further division, 

 forming six to eight cells in which numerous sperm-mother- 

 cells are developed. 



A similar but less complete reduction of the prothallus is 

 seen in the case of the megaspore (c). Its contents are 

 divided, as in Salvinia, into a small mass of protoplasm at 

 one end, and a large quantity of plastic products filling up 

 the rest of its cavity. The protoplasm divides and forms a 



