LIFE HISTORIES IN THE ALGAE WITH SPECIAL REFERENCE TO THE CHLOROPHYTA, PHAEOPHYTA AND RHODOPHYTA

Abstract

SUMMARYThe general essays on this subject which have appeared during the past two or three decades have dealt in the main with types of life history based on the nuclear phases, all algae being classed together. Speculation about the phylogenetical relationship of these types has occupied a prominent place.The aims of this article are somewhat different and are as follows: to consider what is meant by the life history of an alga, to interpret certain characteristic morphological phases present in some life histories, to classify types of life histories recognizable in Chlorophyta, Phaeophyta and Rhodophyta separately, using primarily somatic phases, to indicate the extent of our knowledge. Life history is denned as the recurring sequence of somatic and nuclear phases characteristic of the species under consideration. A life history of an alga should therefore give the reader a comprehensive account of the main features of the succeeding phases of growth and reproduction from a given stage, such as the zygote, until a similar stage is next reached, together with cytological information regarding the place of syngamy and meiosis.In interpreting life histories three considerations are kept in mind: the growth and development of the whole organism, the relation of the organism to its environment, the origin of the more highly specialized life histories which have arisen from the simpler types. The terminology used has been selected carefully to give freedom of thought and to avoid prejudging the interpretation.A nuclear phase is denned as that state of an organism characterized by mitotic divisions all showing the same chromosome number. It then follows that some life histories are monophasic, some diphasic, and so on. Monophasic life histories include haplonts and diplonts, and diphasic life histories diplohaplonts. Derivatives from, and elaboration of, diplohaplonts and their evolutionary significance are considered.A somatic phase is denned as a state of an organism recognizable by a characteristic morphological appearance independent of its chromosome number. Depending on the number of somatic phases recognizable, the life history is said to be mono‐, di‐, or trimorphic. Certain features of some algal life histories are considered in relation to the problem of deciding what is a recognizable somatic phase and it is concluded that: morphologically similar haploid and diploid thalli represent one somatic phase, the hypnozygospore of various Chlorophyta represents a somatic phase, the carposporophyte of the Florideae is a somatic phase. Adelophycean phases are considered, but interpretation in many cases is hampered by the imperfect state of our knowledge.Known monomorphic life histories include diplonts and diplohaplonts, and dimorphic life histories contain both diplohaplonts and haplonts. The only clearly established trimorphic life histories are haplonts.The designation of the individual somatic phases of mono‐, di‐ and trimorphic life histories is discussed. It is shown that the terms gametophyte and sporophyte are of limited use even if it be accepted that gametophytes are thalli, whether haploid or diploid, bearing gametes, and sporophytes are thalli, whether haploid or diploid, bearing spores. Possible derivations arising from di‐ and tri‐ morphic types are discussed.On the basis of the formulated concept of somatic phases a classification of the types of life history in the Chlorophyta, Phaeophyta and Rhodophyta is attempted. Four main types of life history are recognized in each of these algal groups. Symbols are used to designate the types where the necessary information is available and are also used for diagrammatic representation.Lists of comparatively well authenticated examples are given, but certain deviations are noted. This brings out the uniformity found in some taxonomic units and the variety in others.Three characteristic features of the life histories of the most highly developed and specialized algae of the present time, the Laminariales and Desmarestiales (sub‐group C (i) of the Phaeophyta), and the majority of the Florideae (group C), are noted: they are dimorphic, the diplophase is dominant, the sequence of somatic phases is obligatory and shows a constant association with certain nuclear phases. It is concluded that these are important evolutionary achievements and they are further discussed.A study of life histories and the various types of somatic phase recognizable in them leads the investigator to question whether the various types of somatic phase have arisen in the same manner. It also suggests that certain features of the life history of ancestral types, not originally associated with the sexual cycle, may have come gradually to be associated with it and also definite nuclear phases.The dominance of one type of life history among the Chlorophyta of freshwater is in contrast to the various types of life history found among marine algae of the Chlorophyta, Phaeophyta and Rhodophyta.