384 THE SEED AND ITS GERMINATION 



year a million million individuals of that species, the 

 offspring, in the twelfth generation, of a single plant. 

 Many species at the present time are actively spreading 

 and increasing their territory, though the increase is 

 never more than a minute fraction of that which would 

 occur if all offspring survived. Many others are decreas- 

 ing, not holding their own in the struggle for space, 

 light and water. Probably a much larger number of 

 species are more or less holding their own. It is by 

 competition between individuals of the same and different 

 species that this approximate balance is maintained. 



Significance of the Seed in the Evolution o! the Plant 

 World. The ovule< and the seed into which the ovule 

 develops, represent the culmination of the adaptation 

 of the sexual reproductive processes to the conditions 

 of plant life on land, where an external supply of water 

 on the surface of the soil is no longer available. We 

 saw (in Chapter XIV) the preparatory stages which 

 necessarily preceded the appearance of seeds, and which 

 developed many millions of years ago, during the Palaeo- 

 zoic age, in the ancestors of our modern seed plants. 

 These preparatory stages are still represented in the 

 few existing heterosporous Pteridophytes. Instead of 

 uniform spores which germinate to form green free- 

 living plants the prothalli of such plants as ferns 

 which produce the sexual organs, the heterosporous 

 forms produce two kinds of spores, the large megaspores 

 and the small microspores, which germinate to form 

 microscopic prothalli, dependent on the spores, and 

 respectively producing the male and the female gametes. 

 The process of conjugation still depends, however, 

 on external liquid water in which the flagellate 

 male gametes can swim, and this dependence severely 

 restricts the habitats in which such plants can live. 



