THE NATURE OF ALTERNATION, ETC. 325 



the sexual alternation of generations. The latter includes 

 the admittedly homologous alternation in Thallophytes and 

 the antithetic alternation in Mosses and higher plants. Be- 

 tween these, however, Pringsheim recognises a difference 

 of degree only. He regards sporangia, and the sexual 

 organs (antheridia and archegonia in the widest sense) as 

 truly homologous structures, which have proceeded from 

 one another, and considers that this relationship is made 

 manifest " through the representative, correlative succession 

 of generations with spores and of generations with sexual 

 organs. The Thallophyta differ considerably in the relation 

 borne by the sexual and asexual individuals to one another. 

 In some, no alternation is found. The most common case 

 is that a sexual generation alternates with a succession of 

 neutral generations, the last of which again produces the 

 the sexual form. The dimorphic character is, as a rule, 

 exhibited only in the reproductive organs, the generations 

 resembling one another in vegetative structure, and in the 

 possession of subordinate forms of multiplication. In a 

 number of cases, however, the first neutral generation differs 

 more or less widely from those succeeding it. In several 

 genera this generation is reduced to a sporangium, the 

 spores from which may resemble in general appearance 

 those derived from the neutral generation with developed 

 vegetative parts. Oedogonium Coleochcete and Cystopus 

 present in their fruit body, according to Pringsheim, such 

 a reduced first neutral generation. In yet other alga^ {e.g., 

 Splicer op led), this is the only neutral generation found, and 

 the alternation is a definite one between a single sexual and 

 a single spore-bearing generation. The organisms, just 

 mentioned, are compared by Pringsheim, as well as Cela- 

 kovsky, with the simplest Bryophyte sporogonia. But the 

 view, indicated above, of the phylogenetic history of the 

 fruit body, of Coleochcete for example, leads to a conclusion 

 as to the nature of alternation in the archegoniates essentially 

 different from the antithetic theory. The phenomena of 

 Apospory and Apogamy are considered by Pringsheim to 

 support this conclusion that in the Bryophytes and Pteri- 

 dophytes, we have simply a special case of homologous 



