182 Scientific Proceedings, Royal Dublin Society, 



reduction of tho chromosomes in nuclear division seems to me to have been 

 considerably lessened by the discovery of its universality of occurrence in 

 the different forms of life, from Algse and Fungi to Spermatophytes. The 

 reduction is a physiological necessity. Had the water not become dry land, 

 the alternation of generations in ferns might have occurred as it does in 

 Dietyota to-day, but the fern's ancestors would have remained in the thallus 

 stage in both generations. 



Eejecting Potonie's view that the protliallus is an intercalated secondary 

 body, and accepting the view that it is a true gametophyte, sufBcient stress 

 lias not been laid, I think, on its reduced character. Comparison has been 

 confined too much to what is obvious — the gametophyte of existing groups. 

 It is easy to arrange the vascular plants in an ascending series in which the 

 gametophyte becomes less and less differentiated and independent until its 

 extent, not to say existence, in the highest spermatophyte is a subject of dis- 

 cussion. In the same ascending series the archegonia become gradually 

 simpler, more embedded and protected, and finally reduced to tlie essential 

 egg-cell only. The series should be continued downwards from the fern with 

 the thalloid, rather than the foliose forms of tlie Muscinese as guides to the 

 line of descent. The fern prothallus with its emargination depression would 

 become dichotomously lobed, and its archegonia would become more compli- 

 cated and would be stalked or project as in Muscinese. We must assume that 

 at one time the fern-sporophyte was evascular, and the now stable cotyledon 

 seems to give a hint of an ancestor with a dichotomous bifurcation of its parts 

 leading downwards to a stage in which the present marked difference between 

 fern gametophyte and sporophyte, except for mode of reproduction, is lost. 

 The adaptation of an aquatic well-developed thallus to a drier aerial environ- 

 ment offers less difficulty of explanation and accomplishment than the 

 differentiation, ah initio from a zygote, of a sporophyte such as a fern 

 possesses. 



Forhesia cancellata would fit in as an intermediate stage in the differentia- 

 tion of the sporophyte when the plant-body was evascular and dichotomously 

 divided, and when tlie only distinctions between axis and frond were their 

 relative position and the lamination of the frond. Forbesia was " finding its 

 feet," and needed, to stand erect, the mechanical support the sclerotic 

 framework provided. 



