Alternation of generations 137 



genus Chlamydomonas the most primitive form of gamogenesis exists along- 

 side a gamogenesis which has all the essential characteristics of true oogamy. 

 In the Ulotrichales, also, there are all stages between the simplest gamo- 

 genesis, such as occurs in Ulothrix, to the highest type of oogamy such as is 

 found in Cylindrocapsa and Coleochsete. In fact, in the genus Ulothrix the 

 very beginnings of sexuality are observed, since the zoogonidia may be 

 facultative gametes. 



These statements, although very brief, are sufficient to show that in the 

 Green Algae the important upward step from asexual to sexual reproduction 

 has been accomplished independently in more than one group. 



ALTERNATION OF GENERATIONS. Much interest is necessarily attached to 

 the life-histories of the Chlorophycese by reason of the slight indications 

 in various families of what has been suspected to be an alternation of 

 generations. 



From the experimental work which has so far been done on the Green 

 Algae it is possible to conclude with some certainty that in the great majority 

 of forms there is no obligatory succession of phases which can rightly be 

 termed an ' alternation of generations.' The recurrent and alternating phases 

 which are such a marked feature of Archegoniate Plants are at all times 

 associated with distinct cytological differences. The nucleus of the cell of the 

 sporophyte (or asexual generation) has twice as many chromosomes as the 

 nucleus of the cell of the gametophyte (or sexual generation). On the sexual 

 coalescence of the male and female gametes to form the zygote the number of 

 chromosomes is doubled, and in the first division of the zygote, and in all 

 subsequent divisions in the sporophyte generation developed from it, this 

 number (2w) is maintained. The inauguration of the gametophyte generation 

 is by a reduction division, the number of chromosomes being reduced to one 

 half (n), and this number is retained in all subsequent nuclear divisions in the 

 gametophyte. The gametophyte is thus known as the haploid generation in 

 contrast to the diploid or sporophyte generation. The latter is regarded by 

 some authorities as a post-sexual phase interpolated between sexual fusion 

 and chromosome-reduction. 



The question naturally arises as to how far this conception of the sporo- 

 phyte can be applied to any phase in the life-histories of the various Green 

 Algae; and in view of the fact that the first land-plants very probably 

 originated from forms not greatly different from some of the existing Chloro- 

 phycese, there is a further interest in the endeavour to obtain from these 

 life-histories some clue to the origin of the sporophyte of the Archegoniatae. 



The researches of Klebs ('96) and others have shown that in certain of the 

 Chlorophyceaa the various methods of propagation are largely dependent upon 

 external conditions, and that to some extent the desired methods may be 



