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 

 gametophytej which arises from the spore and produces gametes; and the 

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

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

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

 original number at sporogenesis ; t he gametophyte is I herefore t he 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 algae and fungi also, so that the general conception 

 of alternation of generations has been extended to these lower group-. 

 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 Dumber is broken, arc now known. 



the conspicuous examples being found among the fern- and certain 

 angios perms. The very convenient classification of Buch abnormalities 

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

 of the chapter. All dates and t he matter included within square brack* 



have been added by the present author. 



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



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

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



developed otherwise than from ;i po>t -meiot ie Bpore, :i condition known 

 as apospory. 1 



1 [Apogamy in ferns was discovered by Farlo* in 1874. Vpospor} was discovered 

 in mosses by Pringsheim in L876 and in ferns by Druer} in 1884 I ons 



of these phenomena are given bj Winkler 1908 and Strasburgei I •' >.] 



i 1 



