690 TEXTBOOK OF BOTANY 



these structures as well as the egg and sperm contains the monoploid 

 number of chromosomes. The zygote and every subsequent cell of the 

 sporophyte have the diploid number of chromosomes. The endosperm 

 has developed from a nucleus that resulted from the union of three 

 nuclei, and hence each cell of the endosperm has the triploid iiumber 

 of chromosomes. The endosperm is neither gametophyte nor sporo- 

 phyte: it is termed the xeniophyte. 



Comparative summary of life cycles of the great groups of green plants. 

 Several life cycles in each of the great groups of green plants have been 

 considered. Many of the main features of the structure, growth, and 

 reproduction of algae, mosses, liverworts, ferns, and their near relatives, 

 and of seed plants are familiar. If now the fundamental similarities and 

 differences among the life cycles of the great groups of green plants 

 can be visualized, we shall have made a start toward understanding 

 this diverse assemblage of plants as a whole, rather than as individuals 

 or groups. In addition, such an analysis should form a broader basis 

 for an appreciation of the evolution of plants. 



The following statements are true for a majority of the plants belong- 

 ing to the groups indicated. Exceptions may be made, however, to all 

 of them, because simple generalizations can scarcely be formulated for 

 groups containing thousands of species having the greatest diversity of 

 form and habitat. Finding and listing exceptions to these generaliza- 

 tions may prove to be as interesting and profitable as accepting them 

 as universally true. 



1. There are two reproductive phases in the life cycles of most green 

 plants: a gametophyte, in which gametes develop; and a sporophyte, in 

 which spores develop. 



2. In a complete life cycle (or generation) of the plant the gameto- 

 phyte regularly alternates with the sporophyte. Zygotes germinate and 

 sporophytes develop; spores formed in the sporophytes germinate and 

 gametophytes develop. 



3. The gametophyte begins with the spore and ends with the forma- 

 tion of gametes. 



4. The sporophyte begins with the zygote and ends with the forma- 

 tion of spores. 



5. Reduction division occurs during the formation of spores in the 

 sporophyte. 



6. The cells of the gametophyte are characterized by the monoploid 



