46 PLANT-LIFE ON LAND [CH. 



latter are typically aquatic, and their fertilisation is 

 usually carried out through the medium of the water 

 in which they live. We have seen examples of this 

 in Ulothrix and its allies. It therefore seems a 

 probable interpretation of the facts that long ago cer- 

 tain forms of Algae spread from the water to the land ; 

 that they retained signs of their aquatic ancestry both 

 in their method of fertilisation and in some measure 

 as regards the texture of the prothallus. Since the 

 final duty of the prothallus is the formation of the 

 sexual cells, or gametes, it is called the Gametophyte. 

 A second generation or phase, represented by the 

 Fern-plant, underwent special development to fit it 

 for more extended life exposed to the air on land. 

 It has as its final result the production of spores, and 

 it is accordingly called the Sporophyte. A Fern is 

 then an organism, so to speak, with one foot on land, 

 the other in the water. It indicates in the successive 

 events of its life-history a past migration from life in 

 water to life upon land-surfaces. The gametophyte 

 is the conservative phase which retains its aquatic 

 characters, the sporophyte is the innovation, which 

 has assumed characters more suitable to the adopted 

 habitat. Truly the life of such plants may be described 

 as amphibious. 



We have seen that in the Fern the prothallus is 

 relatively small, while the Fern-plant is relatively 

 large. As we rise in the scale of vegetation to the 



