THE EVOLUTION OP PLANTS 13I 



of the consequences of a change from an aquatic to an 

 amphibious habit of life. 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 ferti- 

 lized egg began to develop as a land plant, the chances of 

 fertilization by a sperm swimming in free water became 

 increasingly remote. The perpetuation 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 reproductive body. The larger the 

 number of such spores, the greater the chances of perpetua- 

 tion of the given species. 



105. 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 Uft them up into the 

 air where they would stand a better chance of distribu- 

 tion when dry. This is accompHshed by the sporophyte, 

 producing an abundance of broad, green leaves for food- 

 manufacture, and of roots for absorption of water and 

 minerals in large quantities. From such considerations as 

 these the plant body of the sporophyte is regarded by Bower 

 and his followers as produced by the progressive sterili- 

 zation of tissues originally reproductive. After the for- 

 mation of a vigorous plant body, spores, produced in special 

 regions (sporengia) could be nourished in enormous 

 numbers. 



106. Origin of Vegetative Organs. — On the basis of 

 Bower's theory we are to regard f oUage leaves and branches, 



'See p. 23. 



