BASIS OF ALTERNATION OF GENERATIONS 347 



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

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

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

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

 of chromosomes in the same species (the number characteristic 

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

 alent in their nuclear structure, whatever may be the differences 

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

 somes, and the fertilized egg begins the development of the 

 sporophyte (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 

 nuclear 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 

 24 chromosomes, and the gametophyte phase only 12. The 

 chromosomes have been counted in more than fifty different 

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

 sporophytes have normally double the number of chromosomes 

 of their respective gametophytes. 



Spore formation at the end of the sporophyte generation is a 

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

 double number of chromosomes is reduced by half. The spores 

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

 tion of the chromosomes is effected by processes too complicated 

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

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

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

 thus fundamental reasons for the identical methods of spore for- 

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

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

 in 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. 



