WAHL: ALTERNATION OF GENERATIONS 3 



fact that the number of spores formed in each mother cell is four appears 

 to have no important morphological significance. It has no connection with re- 

 duction phenomena which — take place before these divisions." Obviously, our 

 knowledge of chromosome function and behavior has increased greatly since 

 the above excerpts were first written. It seems equally obvious, however, that 

 this knowledge has not been applied, or has been indifferently applied, in an 

 interpretation of the relation of these phenomena to the sexual life cycle of 

 plants. 



It should by now be apparent that the process of sexual reproduction in- 

 volves not only the fusion of gametes (syngamy) by which the zygote receives 

 the sum of the chromosomal components of the two gametes, but that the pro- 

 cess by which the chromosome number is reduced (meiosis) is an integral 

 part of the sexual life cycle. In animals generally and in those plants (Fucus, 

 diatoms, Siphonales) in which meiosis shortly precedes syngamy, this rela- 

 tionship is obvious. In most green algae and in other Thallophyta in which 

 meiosis follows syngamy without the intervention of a growth phase or alter- 

 nating generation, the relation should be equally obvious. In those plants in 

 which a growth phase intervenes between syngamy and meiosis, that is, in 

 which an alternation of generations occurs, the separation of syngamy and 

 meiosis in point of time seems to have been instrumental in the frequent fail- 

 ure to associate the processes as integral parts of the sexual mechanism. 



The difficulties which arise from the dissociation of the meiotic processes 

 and syngamy and the association of meiosis with asexual reproduction have 

 their beginning, as far as elementary teaching is concerned, in a consideration 

 of the reproductive processes in the lower plant groups. In many algae, vege- 

 tative (asexual) reproductive cells such as zoospores, aplanospores, etc., are 

 produced by the haploid plant body while the same plant body produces gam- 

 etes. Following the fusion of gametes, the next reproductive process in the life 

 cycle of many common algae is the meiotic division of the nucleus of the zygote, 

 a process which results in the production of four spores. Since asexual reproduc- 

 tion has consistently been considered as including all reproductive processes not 

 directly initiated by a fusion of cells and since it refers especially to the production 

 of spores, the tendency here is purposely or passively to homologize the vegeta- 

 tively-produced spores with the spores produced as a result of meiosis. The vege- 

 tative production of spores is exactly comparable as a reproductive method to the 

 production of gemmae by liverworts and mosses and to the production of gem- 

 mae, bulblets and the host of other vegetative reproductive devices by vascular 

 plants. The reproductive methods are entirely comparable regardless of the fact 

 that we are here comparing processes of the gametophyte with those of the sporo- 

 phyte; they all consist of the rejuvenation of an individual by the initiation of 

 growth from some more or less specialized portion of the vegetative plant body. 

 In some algae in which an alternation of generations occurs (e.g. Cladophora) 



