A GENETICAL INTERPRETATION OF ALTERNATION OF GENERATIONS
It is necessary to realize the limitations of cytological data as criteria of the generations, since some workers have unconsciously tended to regard the two generations as separate entities rather than developmental phases of a single one.
Abstract
A REVIEW of the literature dealing with this fascinating subject would be superfluous here, primarily because excellent summaries have been provided by others (e.g. Bower, I935), and secondarily because this interpretation is based upon no pre-existing theories. There has been much controversy with respect to the adequacy of cytological data as criteria of the generations. If it be supposed as a hypothesis that the diploid number of chromosomes is the fundamental character of the sporophyte, then surely the invalidity of such a hypothesis would be demonstrated by the existence of haploid sporophytes. Such haploid sporophytes and similarly diploid gametophytes are of course common, and it is therefore necessary at least to realize the limitations of cytological data as criteria of the generations. It is important to understand what is really implied by the symbol n. Terminologically it symbolizes a certain number of chromosomes, but fundamentally it implies a number of genes, and cytogenetical evidence indicates that these are responsible for the production of all the characters of an organism, though their effect may be modified with respect to the environment in which they react. In a plant such as a moss the gametophyte possesses certain characters which are absent in the sporophyte and vice versa. Since each character is but the effect of a gene or genes, it would appear that each generation possesses certain genes which are absent in the other. However in the self-fertilization of a pure line the diploid number of chromosomes implies only the duplication of genes and not the inclusion of new ones, so that all the characters both of the sporophyte and gametophyte are represented by genes included in the symbol n. It is important to grasp this fact, since some workers have unconsciously tended to regard the two generations as separate entities rather than developmental phases of a single one. The haploid complement of genes thus comprises certain units which produce their effect early, i.e. in the gametophyte and others which