CHAPTER XIII 



APOGAMY, APOSPORY, AND PARTHENOGENESIS 

 APOGAMY AND APOSPORY 



The life cycle in all bryophytes and vascular plants is characterized 

 by a regular alternation of two well marked phases or generations: the 

 gametophyte, which arises from the spore and produces gametes; and the 

 sporophyte, which arises from the fusion product of two gametes and 

 produces spores. In such a normal life cycle the number of chromosomes 

 in the nuclei is doubled at the union of the gametes and reduced to the 

 original number at sporogenesis ; the gametophyte is therefore the haploid 

 generation and the sporophyte the diploid generation, their limits being 

 marked by the two cytological crises, fertilization and reduction. Such 

 an alternation of haploid and diploid phases has been discovered in 'the 

 life cycles of many algas and fungi also, so that the general conception 

 of alternation of generations has been extended to these lower groups. 

 This, however, is not the place for a discussion of the homologies implied. 

 It should be added that gametophyte and sporophyte may arise not only 

 from each other, but either generation may also multiply by vegetative 

 means. 



Many instances in which the above typical life cycle is departed from, 

 and in which the correlation between the alternation of two generations 

 and periodic changes in chromosome number is broken, are now known, 

 the conspicuous examples being found among the ferns and certain 

 angiosperms. The very convenient classification of such abnormalities 

 drawn up by Vines (1911) is given as the basis for the present portion 

 of the chapter. All dates and the matter included within square brackets 

 have been added by the present author. 



"In the first place, the sporophyte may be developed either after an 

 abnormal sexual act, or without any preceding sexual act at all, a con- 

 dition known as apogamy. In the second, the gametophyte may be 

 developed otherwise than from a post-meiotic spore, a condition known 

 as apospory. 1 



1 [Apogamy in ferns was discovered by Farlow in 1874. Apospory was discovered 

 in mosses by Pringsheim in 1876 and in ferns by Druery in 1884. General discussions 

 of these phenomena are given by Winkler (1908) and Strasburger (19096).] 



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