EVOLUTION OF THE LAND PLANT 107 



As the spore-bearing plant or sporophyte grew larger 

 and by its erect position became carried away from the 

 ground, the separation of the two kinds of prothalli 

 developed from the spores rendered fertilisation by 

 means of free-swimming sperms increasingly difficult. 

 The larger spores, 

 too, tended to stay 

 longer and longer nrch 

 in their sporangia, 

 so that the time for 

 fertilisation became 

 shortened. This led 

 to the establish- 

 ment of a new 

 method of securing 

 fertilisation which 

 we find exhibited 

 by the flowering 

 plants. The pro- 

 thalli - bearing ova 

 became entirely en- 

 closed in the large 

 spore, and those 

 bearing sperms be- 

 came entirely fila- 

 mentous. Instead 

 of the sperm 

 travelling to the ovum, the small spore itself was brought 

 by various ways to the immediate neighbourhood of the 

 large one, so that the prothalli were developed in close 

 proximity to each other. The filamentous prothallus of the 

 small spore bored its way into the surroundings of the large 

 spore and the latter remained altogether in its sporangium. 

 Fertilisation consequently came about by means of a 

 tubular outgrowth of the little spore, instead of a free- 

 swimming sperm. If we compare the two methods, both 



FIG. 43. Germination of megaspore of Selagi- 

 nella, showing prothallus almost entirely 

 inside the spore, which has opened at the 

 apex, arch, archegonia; oos, ovum; em, 

 young embryos at different stages of de- 

 velopment. 



