ALTERNATION OF GENERATIONS. 



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b. The problem which is met by the theory of alternation of generations in the form 

 :m which it has now been presented, is to refer the most important stages in the 

 [history of development of all plants to a single scheme which is illustrated most clearly 

 in the cases of Muscineae and Filices, where Hofmeister first discovered this alternation 

 in 185 1. The same botanist was also the first to explain the development of the seed 

 [in Gymnosperms by the alternation of generations in Lycopodiaceae, and hence to 

 [compare it with the same phenomenon in Filices and Muscineae. At the present time 

 [our knowledge of the development of Thallophytes has made so much progress that 

 [it is possible to determine what are its main features, and to compare them with those 

 [of Muscineae and vascular plants. This comparison, which has only been briefly 

 [indicated above, will be followed out more in detail in the sequel, and will lead to 

 [the result that Thallophytes may also be included in the scheme under which the 

 )ther classes are comprised, the first stage of development closing with the formation 

 [of sexual organs, from which proceeds the second generation, essentially different 

 [from the first, and closing with the production of true spores. It will therefore show 

 [that the development of all plants which possess sexual organs may be divided into 

 two stages which correspond in all essential points to the two generations in the life- 

 [history of a Fern ; and that there is, therefore, in the whole vegetable kingdom, 

 [only one type of alternation of generations so far as it is brought about by sexual 

 : organs. 



