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SCIENCE 



[N. S. Vol. XXXIX. No. 1000 



IV. THE DISCOVERY OF THE ALTERNATION OP 

 GENERATIONS IN PLANTS, 1851- 



The fact that the sexual cells of the 

 higher plants are produced on a plant body 

 or individual distinct from that which 

 forms the asexual reproductive cells; and 

 that in the normal life cycle the one type of 

 individual arises from, and later gives rise 

 to, an individual of the other type, must 

 be regarded as one of the most significant 

 features of the evolution of plants yet dis- 

 covered. One of the chief general results 

 of the magnificent work of Hofmeister was 

 the discovery of this regular alternation 

 of a sexual and an asexual generation, not 

 only in the life history of the mosses and 

 ferns, but also in that of the seed plants. 

 Hofmeister states this result clearly in the 

 Vergleichende Unters-uchungen and makes 

 it apply still more broadly in a brilliant 

 generalization published in the "Higher 

 Cryptogamia. " There he says (p. 439): 



The phenogams, therefore, form the upper ter- 

 minal link of a series, the members of which are 

 the Coniferae and Cyeadeffi, the vascular crypto- 

 gams, the MuscineEB and the Characese. These 

 members exhibit a continually more extensive and 

 more independent vegetative existence in propor- 

 tion to the gradually descending rank of the gen- 

 eration preceding impregnation, which generation 

 is developed from reproductive cells cast off 

 from the organism itself. 



Since Hofmeister's day detailed investi- 

 gations by many workers have fully eon- 

 firmed Hofmeister's conclusion. They have 

 shown the essential homology, not only of 

 the spore-producing organs, and the one or 

 two kinds of spores produced in them, but 

 also of the structures arising from these 

 spores, throughout all cormophytes, from 

 the mosses upward. 



In the studies of the algae that followed 

 immediately after Hofmeister's work, in- 

 vestigators of these plants sought in them 

 for some evidence of that regular alterna- 

 tion of sexual and asexual phases that had 



been demonstrated in higher plants. 

 Pringsheim (1856, p. 14) one of the ablest 

 of these students of the algse, at first re- 

 garded the multicellular body, formed at 

 the germination of the oospore of CEdo- 

 gonium and Coleochcete, as an asexual 

 phase, comparable with the simple sporo- 

 phyte of the liverwort Biccia. Celakowsky 

 (1886) distinguishes as homologous alter- 

 nation those cases, in algae, like TJlothrix or 

 OEdogonium, where the gamete-producing 

 generation seemed capable of zoospore pro- 

 duction also. The constant and regular 

 alternation of the archegoniates and seed 

 plants he called antithetic alternation. 

 Pringsheim (1877) found that moss pro- 

 tonemata form from cuttings of the seta 

 of the sporophyte as well as from bits 

 of the gametophyte. From this fact, and 

 from Farlow's discovery (1874) that a 

 sporophyte of the fern, Pteris cretica, may 

 arise directly from the prothallus, without 

 the fertilization or even the formation of 

 an egg, Pringsheim concluded that both 

 generations of the archegoniates are really 

 identical. He says (1877, p. 6) : 



I believe the moss sporogonium stands to the 

 moss plant in the same relation that the spor- 

 angiiun-bearing Saprolegnias do to the oogondum- 

 bearing plants of this species, ... I therefore turn 

 against this interpretation of the fruit genera- 

 tion of the thallophytes in general, and especially 

 against this interpretation of the sexual shoot 

 generation of the Florideee and Aseomycetes. 

 . . . The cystocarp is evidently not a separate 

 individual but part of the sexual plant that pro- 

 duces it. 



The antithetic view was reasserted, how- 

 ever, especially by Celakowsky (1877) and 

 Bower (1890), both of whom emphasized 

 the suggestion of A. Braun (1875) that the 

 sporophyte is a new thing phylogenetieaUy. 

 Bower holds that the types of sporophyte 

 found in the archegoniates have arisen by 

 the amplification of the zygote, with the 

 sterilization for vegetative functions of 



