BASIS OF ALTERNATION OF GENERATIONS 347 



ie liverwort (Anthoceros, Sec. 290) have only 4 and the fern 

 (Osmunda) 12. The most important feature of the process of 

 srtilization is the union of the two gamete nuclei, that of the 



3rm with that of the egg. These nuclei have an equal number 



chromosomes in the same species (the number characteristic 

 jf the gametophyte), and the egg and sperm are therefore equiv- 

 lent in their nuclear structure, whatever may be the differences 



their size. This nuclear fusion doubles the number of chromo- 

 )mes, and the fertilized egg begins the development of the 

 3orophyte (when present) with twice as many chromosomes as 

 the gametophytes which produced the eggs and sperms. 



The double number of chromosomes appears in all of the 

 luclear divisions throughout the development of the sporo- 

 phyte up to the time of spore formation. Thus the sporophyte 

 phases of Polysiphonia have about 40 chromosomes, the sporo- 

 phyte of Anthoceros 8, and Osmunda 24. The lilies have 

 J4 chromosomes, and the gametophyte phase only 12. The 

 chromosomes have been counted in more than fifty different 



ids of plants, mostly seed plants, and it is established that 

 sporophytes have normally double the number of chromosomes 



their respective gametophytes. 



Spore formation at the end of the sporophyte generation is a 

 rery significant period in the life history, for at this time the 

 louble number of chromosomes is reduced by half. The spores 

 ive then the original number of the gametophyte. The reduc- 

 tion of the chromosomes is effected by processes too complicated 



be described here, but the formation of the asexual spores in 

 s of four, called tetrads (see Figs. 204, 245, 258, 289, 298, 

 J02, 304), is rather characteristic of the phenomenon. There are 

 ms fundamental reasons for the identical methods of spore for- 

 lation in the bryophytes and pteridophytes, and, as will appear 

 iter, for the methods of pollen formation and the embryo sac 



the seed plants. For the same reasons, groups of four spores, 

 (tetraspores), are developed at the end of the sporophyte genera- 

 tion in the red algae. 



