CH. XII] 



THE SELAGINELLAS 



507 



The microspores develop a prothallus of 

 cell, the remainder of their contents forming a 

 large antheridium containing the sperm-cells, which are 

 five swimming and biciliate. The megaspore forms a multi- 

 cellular prothallus mostly inside the spore, though it breaks 

 the coat and grows somewhat outward, producing the 

 archegonia on the exposed part. After fertilization, which 

 in some kinds occurs while the spores are still on the plants, 

 the egg cell grows to an embryo which has a suspensor and 

 remains for some time buried in the 

 prothallus, as in case of the Seed 

 plants. In germination the foot, one 

 of the four primal subdivisions of the 

 embryo as in Ferns (page 497), often 

 remains in the prothallus after the root 

 and shoot have issued, much as do 

 the cotyledons in some Seed plants. 

 Indeed it is possible that this haus- 

 torial foot, and not the first leaves, is 

 the morphological prototype of the 

 cotyledons. 



Related to the Lycopods in their 

 mode of reproduction, though very 

 different vegetatively, is Isoetes, the 

 Quillwort (Fig. 360), which grows in marshes and shoal 

 ponds. The plants are tufted, with awl-shaped leaves, at 

 the bases of which are .borne the heterosporous sporangia. 



The homologies of these parts are all perfectly clear. The 

 Selaginella plant is the sporophyte, but there are two game- 

 tophytes, male and female, which are not independent plants 

 as in the true Ferns, but dependent for their nutrition upon 

 food supplied by the sporophyte. This condition continues 

 also in the Spermatophytes, and stands in great contrast 

 to that in forms below the Ferns, where the gametophyte 

 is the prominent vegetative plant, and the sporophyte is 

 dependent for nutrition upon it. 



FIG. 360. Isoetes lacus- 

 tris; X J. (From Kerner.) 



