THE NATURE OF ALTERNATION, ETC. 335 



•one of the most important additions to our knowledge of 

 the facts of alternation. This is manifested by the number 

 of chromosomes which are distinguishable in the dividing- 

 nucleus being twice as great in the cells of the sporophyte 

 as in those of the gametophyte. This has now been 

 established for representatives of the Bryophyta, Pterido- 

 phyta, Gymnosperms and Angiosperms, and, although the 

 results need to be extended, may reasonably be assumed to 

 hold throughout these groups. The increase in number of 

 chromosomes takes place as the result of the sexual fusion ; 

 the double number is retained in the vegetative cells of the 

 sporophyte ; the reduction occurs in the spore mother cells. 

 The existence of the double number of chromosomes in the 

 cells of the sporophyte affords support to the antithetic 

 theory of alternation, in that the spore-bearing generation 

 appears as an interpolation, not merely between two succes- 

 sive gametophytes, but between the sexual fusion and the 

 reduction in number of the chromosomes. The bearing of 

 the facts known as to this periodic reduction of chromo- 

 somes upon the nature of alternation has been considered 

 by Strasburger 1 in two important papers. His general con- 

 clusion may be given: "The morphological cause of the 

 reduction in number of the chromosomes and of their 

 equality in number in the sexual cells, is in my opinion 

 phylogenetic. I look upon these facts as indicating a return 

 to the original generation from which, after it had attained 

 sexual differentiation, offspring was developed having a 

 double number of chromosomes." 



It thus becomes of great importance to ascertain whether 

 corresponding phenomena take place in those Thallophyta 

 which approach the Bryophyta most nearly in their alterna- 

 tion of generations. The facts are still unknown for such 

 Algae as Oedogonium and Coleochcete and in the only Alga 

 yet accurately investigated (Fucus) ' 2 the reduction takes 

 place just before the development of the sexual cells. 

 Fucus, however, cannot be regarded as bearing directly on 



1 Strasburger (2), (3). See also the earlier paper by Overton. 



2 Farmer and Williams. See also Strasburger, Jahrb. Wiss. Bot., 



'897, P- 35 1 - 



