WAHL: ALTERNATION OF GENERATIONS 5 



In a more recent text (Weather wax, 1942), one that is also intended for 

 use in a shorter course, the author considers the production of meiospores as 

 part of sexual reproduction. "The complete sexual life cycle consists of two 

 generations, the gametophyte and the sporophyte, which follow each other 

 in alternate sequence." (p. 196.) 



It is refreshing to note that this viewpoint which considers a life cycle in- 

 volving an alternation between a gametophytic phase and a sporophytic phase 

 as a sexual life cycle is at least partially maintained in one of the more exten- 

 sive modern elementary texts (Transeau, Sampson and Tiffany, 1940). In 

 discussing reproduction in Ulothrix the authors state : "The special method 

 of vegetative multiplication by means of either motile or non-motile spores 

 which are formed without a previous union of gametes is often termed asexual 

 reproduction. The related series of processes including the formation of gam- 

 etes, their subsequent union, and the development of the resulting zygote into 

 motile spores from which new filaments develop are referred to as the sexual 

 reproduction of the algae." The life cycle of flowering plants is also considered 

 a sexual life cycle and by inference the sporophyte is as much a sexual individ- 

 ual are are the gametophytes. It is a bit disconcerting, however, to find under 

 the discussion of liverworts the heading "The sporophyte and asexual repro- 

 duction" leading to the inference that here the production of meiospores is not 

 part of sexual reproduction and that the sporophyte is an asexual generation. 



The above mentioned cases constitute the exception. From a consideration 

 of other books, it is apparent that the student is in most cases introduced to the 

 idea that following sexual reproduction (fusion of gametes, or syngamy) an 

 asexual reproductive process (production of spores by meiosis) occurs. This 

 must be expanded in the higher plants to include the idea of alternating sexual 

 and asexual generations with their concomitant methods of sexual and asexual 

 reproduction. The elaboration of the idea of alternating sexual and asexual 

 reproductive methods by Coulter (1914) has probably been of greater im- 

 portance in perpetuating this concept than any other work. 



Several disadvantages of this viewpoint may here be pointed out. In the 

 first place, the concept of a life cycle involving an alternation of sexual and 

 asexual reproduction and sexual and asexual generations is inherently more 

 difficult for the beginner to grasp than the same series of events explained in 

 terms of a continuous process as a sexual life cycle. Let it not be thought, how- 

 ever, that the present author would condone any presentation which merely 

 substitutes simplicity for correctness. The greater simplicity of considering the 

 life cycle a sexual cycle has been demonstrated in class work. The assumption 

 that the viewpoint is correct is based, of course, not on new facts but on an eval- 

 uation of accumulated ideas. 



In the second place, the consideration of the sporophyte as an asexual gen- 

 eration and the production of spores by meiosis as an asexual process leads 



