THE EVOLUTION OF PLANTS 573 



fact that the most primitive sporophytes of the lower 

 liverworts consist almost entirely of "fertile" (i.e., re- 

 productive) cells, and that the relative amount of vege- 

 tative or sterile tissue gradually increases (as we pass to 

 more highly organized forms) by a progressive sterilization of 

 fertile tissue. This progressive sterilization accompanied 

 a change in the habitat of plants from water to dry land. 



496. Consequences of an Amphibious Habit of Life . 

 The life history of the fern affords a concrete illustration. 

 The gametophyte is semi-aquatic in habit, and the method 

 of fertilization is purely aquatic, the sperm being unable 

 to reach the egg except by swimming through free water. 

 But, when the fertilized egg began to develop as a land 

 plant, the chances of fertilization by a sperm swimming 

 in free water became increasingly remote. The perpetua- 

 tion of the species, and the multiplication of individuals 

 could be insured only by the formation of a large number of 

 reproductive bodies (spores), capable of distribution by 

 wind in dry conditions, and each able to reproduce its 

 kind independently, without fusion with another repro- 

 ductive body. The larger the number of such spores, the 

 greater the chances of perpetuation of the given species. 



497. Consequence of Enormous Spore -production. 

 But the formation of a large number of spores requires a 

 vigorous plant body to supply them with an abundance 

 of water and nourishment, and to lift them up into the 

 air where they would stand a better chance of distribution 

 when dry. This is accomplished by the sporophyte, 1 

 producing an abundance of broad, green leaves for food- 



1 "The fern, as we normally see it, is an organism with, so to speak, 

 one foot in the water, the other on the land." Bower, F. O., "The Origin 

 of a Land Flora," p. 82, 



