THE NATURE OF ALTERNATION, ETC. 327 



(ovum) does not give rise to a prothallus or the sporogonium 

 stalk to a new sporogonium. The threefold alternation, 

 which as the title of this paper indicates is recognised in 

 the vegetable kingdom, may be mentioned in conclusion. 

 The forms are: (1) Homologous alternation in the Thallo- 

 phyta between two or more Protophyt generations. (2) 

 Antithetic alternation between Protophyt and Antiphyt in the 

 Mosses and Vascular Plants. (3) Homologous alternation 

 between two or more Antiphyt generations, i.e. alternation 

 of shoots in vascular plants, and especially in phanerogams. 



Summing up the state of the question at this date (1878) 

 we find that, owing in great part to the work of Pringsheim 

 and Celakovsky, the morphological problem presented by 

 the alternation of generations in archegoniate plants had 

 been clearly recognised. It was seen that the true nature 

 of alternation in Mosses and Ferns was only to be ascer- 

 tained by arriving at correct views of the manner in which 

 these groups had descended from lower forms such as those 

 represented by the Thallophyta. On the one hand this 

 might have taken place by the further development of a 

 generation equivalent to the sexual generation, the indivi- 

 duals of which had become more or less reduced and 

 remained in connection with the parent plants. On the 

 other hand the elaboration of the product of fertilisation 

 might have been an entirely new development, the result 

 of which did not represent what had at any period been an 

 independent generation. The further development of this 

 interpolated generation with the appearance of vegetative 

 organs might have proceeded by some of the spore-pro- 

 ducing cells becoming sterile. On the first theory the parts 

 of the second generation as well as the generation itself 

 might exhibit homologies with the sexual generation on 

 the latter homology (in the sense of homology by descent) 

 is out of the question. It was as a subordinate extension 

 of the homologous theory that Pringsheim 1 considered 

 actual homology to exist between the seta of a moss capsule 

 and the stem of the leafy plant. 



Deferring until afterwards the consideration of theoretical 



1 Pringsheim (5). 



