321 



©ngtnal ^rttcits. 



ON ALTERNATION OF GENERATIONS IN THE 

 THALLOPHYTES. 



By Sydney H. Vines, M.A., D.Sc, F.L.S., Fellow and Lecturer 

 of Christ's College, Cambridge. 



The discovery of the fact that an alternation of generations, 

 comparable to that already known in the case of certain animals, 

 takes place in the life -history of certain plants, is due chiefly to 

 the labom-s of Hofmeister/'' So far back as 1849 he pointed out 

 that the i3rot]iallinm of the Vascular Cryptogams is morphologically 

 equivalent to the Moss-plant ; that a Fern, a Lycopod, or a PJiizo- 

 carp, is the homologue of the Moss-fruit ; and further, that in both 

 Mosses and Ferns the asexual is interrupted by a sexual repro- 

 duction, this interruption occurring at an earlier stage in Ferns 

 than in Mosses, and that the sexual and asexual generations 

 regularly alternate. 



So sooii as these views were generally accepted, attempts were 

 made to ai^ply them to the life -histories of the Phanerogams on the 

 one hand, and of the Thallophytes f on the other. Such an 

 apphcation is rendered difficult in the case of the former group, by 

 the very intimate connexion of the two generations in the ovule, 

 which makes their exact distinction a matter of some uncertainty ; 

 in fact, in spite of the considerable recent additions to our 

 knowledge of the structure of the ovule and of the process of 

 fertilisation, this question is still under discussion : in the case of 

 the latter grou^^ it is rendered difficult by the more or less 

 complete index^endence of the two generations, and by the frequent 

 repetition of the one before the recurrence of the other. Some 

 Thallophytes, however, viz. the ConjugatcB, Fxicacea, and Characece, 

 are generally considered not to exhibit any alternation of genera- 

 tions. It is the object of this paper to discuss the application to 

 the Thallophytes of this doctrine, and to inquire in how far such 

 an application is justifiable. 



Before proceeding to do this it will be well to give a brief 

 sketch of the life -history of a Moss, which may serve in some sort 

 as a standard for comparison. Calling the sj)orogonium (the 

 asexual generation or sporophore) the first generation, we find that 



* Ueb. die Fruchtbildung iind Keimun.i? der hoheren Kryptogamen, ' Bot. 

 Zeit.,' 1849, No. 45. — Vergl. Unters. 1851. On the Higher Cryptogainia, 

 ' Eay. Soc.,' 18ti2. Zur Uebersicht der Geschichte von der Lehre der Ptlanzen- 

 befruchtung, ' Flora,' 1867, p. 120. 



+ For instance, Pringsheim, Ueb. die Befruchtung und den Genevations- 

 wechsel der Algen, ' Monatsber. der Berl. Akad.,' Mai, 1800: — Braun, Ueb. 

 Parthenogenesis bei Pflanzen, ' Abhandl. d. k. Berl. Akad.,' 1856. 



X. s. VOL. 8. [NoveaMber, 1871).] 2 t 



