EVOLUTION OF THE PLANT KINGDOM 425 



ing only one new plant, the zygote now indirectly produces, through the 

 development of the sporophyte, a large number of new plants. (2) An 

 increase in the possible range of variation. Where the zygote undergoes 

 meiosis only two genetically different lines of descent are possible, since 

 the segregation of genes takes place during the first reduction division. 

 The development of a sporophyte results in meiosis in many spore mother 

 cells, all descended from the same zygote, and so makes possible a great- 

 many new chromosome combinations. The advantages of alternation of 

 generations are proportional to the size and length of life attained by the 

 sporophyte and account, at least in part, for its dominance over the game- 

 tophyte in vascular plants. 



The evolution of the sporophyte in the higher plants has been marked by 

 a prolongation, through vegetative growth, of the interval between fer- 

 tilization and meiosis. The sporophyte of Riccia represents a primitive 

 condition in that its growth period is short and nearly all its cells produce 

 spores. Throughout bryophytes and pteridophytes progressive steriliza- 

 tion of potentially sporogenous tissue has resulted in an elaboration of 

 vegetative structures. At the same time, not only is relatively less and 

 less tissue devoted to spore production, but it appears later and later in 

 the life history. 



As the sporophyte has achieved independence and become the dom- 

 inant generation in all vascular plants, the gametophyte has undergone a 

 progressive decline. This has been accentuated by the development of 

 heterospory, first seen in the pteridophytes. In spermatophytes the 

 gametophyte has become not only greatly reduced structurally, but actu- 

 ally is parasitic on the sporophyte, thus reversing conditions in the bryo- 

 phytes, where the sporophyte is parasitic on the gametophyte throughout 

 its entire existence. 



