THE NATURE OF ALTERNATION, ETC. 323 



three forms of alternation of shoots are distinguished : (1) 

 the alternating shoots owe their distinctness to phyllo- 

 morphism ; (2) the leafy shoot is produced from a thallome, 

 e.g., moss-plant from protonema ; (3) the second shoot 

 generation comes into existence as the result of a sexual 

 act (Ascomycetes, Basidiomycetes, Floi'idece.) This third 

 form is termed antithetic alternation of shoots. In contrast 

 to alternation of shoots is placed the alternation of bionts ; 

 two essentially different kinds of this are distinguished as 

 antithetic and homologous alternation. In the former, seen 

 in Muscineae and Vascular Cryptogams, two fundamentally 

 distinct generations are present, the asexual either being 

 immediately itself a fruit body, or as its ultimate aim pro- 

 ducing fruits and spores. The existence of independent 

 sexual and asexual plants, so commonly seen in Thallo- 

 phytes, constitutes homologous alternation. If the alterna- 

 tion of asexual and sexual generations in Algae be repre- 

 sented by AB, AB the antithetic alternation will be BC, 

 BC. Among Algae Coleochcete and Oedogonium possess 

 the three generations A, B, C. The greater definiteness of 

 the succession of the alternating antithetic generations is 

 referred to the fact that the sexual cells and spores are 

 strictly connected with their respective generations. " The 

 protophyt is never able to produce spores as well as sexual 

 cells, the antiphyt never sexual cells besides spores." In 

 homologous alternation on the other hand the same indi- 

 vidual may bear sexual and asexual reproductive organs. 

 The difference is explained when it is remembered that the 

 antiphyt was originally a fruit generation, and only later 

 took on vegetative functions in the Vascular Cryptogams. 

 In the concluding part of this paper, the views of Stras- 

 burger mentioned above are critically examined. 



Alternation of generations was again considered by 

 Braun in 1875. 1 The view taken agrees on the whole 

 with that of Celakovsky, though the author inclines to 

 regard the fruit generation, of Coleochcete at least, as 

 homologous with the thallus of the sexual generation. He 

 recognises the importance of antithetic alternation, al- 



1 Braun (3). 



22 



