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 A L G M they occupy among Algae and also the remarkable uniformity in their reproductive processes, it is clear that, as is the case among Archegoniatce, the product of the sexual act never germinates directly into a plant which gives rise to the sexual organs. Even among Bangiacece the carpospores arise from the fertilized cell by division, while in all other Rhodophycece the oospore, as it may be called, gives rise to a filamentous structure, varying greatly in its dimensions, epiphytic, and to a large extent parasitic upon the egg-bearing parent plant, and in the end giving rise to carpospores in the terminal cells of certain branches. There is here obviously a certain parallelism with the case of Bryophyta, where the sporogonium arising from the oospore is epiphytic and partially parasitic upon the female plant, and always culminates in the production of spores. Not even Biccia, with its rudimentary sporogonium, has so simple a corresponding stage as Bangia, for, while there is some amount of sterile tissue in Biccia, in Bangia the oospore completely divides to form carpospores. Excluding Bangiacece, however, from consideration, the Eufloridece present in the product of the development of the oospore like Bryophyta a structure partly sterile and partly fertile. There is, nevertheless, this important difference between the two cases. While the spore of Bryophyta on germination gives rise to the sexual plant, the carpospore of the Alga may give rise on germination to a plant bearing a second sort of asexual cells, viz., the tetraspores, and the .sexual plant may only be reached after a series of such plants have been successively generated. It is possible, however, that the tetraspore formation should be regarded as comparable with the prolific vegetative reproduction of Bryophyta, and in favour of this view there is the fact that the tetraspores originate on the thallus in a different way from carpospores with which the spores of Bryophyta are in the first place to be compared; moreover, in certain Nemalionales the production of tetraspores does not occur, and the difficulty referred to does not arise in such cases. Altogether, it is difficult to resist the conclusion that Floridece present in a modified form the same fundamental phenomenon of alternation of generations as prevails in the higher plants. Among Phceophycece it is well known that the oospore of Fucacece germinates directly into the sexual plant, and there is thus only one generation. Moreover, it is known that the reduction in the number of chromosomes which occurs at the initiation of the gametophyte generation in Pteridophyta occurs in the culminating stage of Fucus, where the oogonium is separated from the stalk-cell, so that, unless it be contended that the Fucus is really a sporophyte which does not produce spores, and that the gametophyte is represented merely by the oogonium and antheridium, there is no semblance of alternation of generation in this case. The only case among Phceophycece which has been considered to point to the existence of such a phenomenon is Cutleria. Here the asexual cells are borne upon the so-called Aglaozonia reptans and the sexual cells upon the plants known as Cutleria. The spores of the Aglaozonia form are known to give rise to sexual plants, and the oospore of Cutleria has been observed to grow into rudimentary Aglaozonia. It is probable, however, that there is nothing more here than strongly contrasted growth-forms. Among Chlorophycece, it is often the case that the oospore on germination divides up directly to form a brood of zoospores. In Coleochcete, this seems to be preceded by the formation of a minute parenchymatous mass, in each cell of which a zoospore is produced. In Sphceroplea, it is only at this stage that zoospores are formed at all; but in most cases, such as (Edogonium, Ulothrix, Coleochcete, similar zoospores are produced again and again upon the thallus, and the product of the oospore may be regarded as merely a first brood of a series. It

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has been held by some, however, that the first brood corresponds to the sporophyte generation of the higher plants, and that the rest of the cycle is the gametophyte generation. Were the case of Sphceroplea to stand alone, the phenomenon might perhaps be regarded as an alternation of generations, but still only comparable with the case of Bangia, and not the case of the Floridece. But it is difficult to apply such a term at all to those cases in which there intervene between the oospore and the next sexual stage a series of generations, the zoospores of which are all precisely similar. The difficulty of tracing the relationships of Algae is largely due to the inadequacy of our knowledge of the conditions under which they pass through the critical stages of their life-cycle. Of the thou- ^rphism. sands of species which have been distinguished, relatively few have been traced from spore to spore, as the flowering plants have been observed from seed to seed. The aquatic habit of most of the species and the minute size of many of them are difficulties which do not exist in the case of most seed-plants. From the analogy of the higher plants observers have justly argued that when they have seen and marked the characters of the reproductive organs they have found the plant at the stage when it exhibits its most noteworthy features, and they have named and classified the species in accordance with these observations. While even in such cases it is obvious that interesting stages in the life of the plant may escape notice altogether, in the cases of those plants the reproduction of which is unknown, and which have been named and placed on the analogy of the vegetative parts alone, there is considerable danger that a plant may be named as a distinct species which is only a stage in the life of another distinct and perhaps already known species. To take an example, Lemanea and Batrachospermum are Floridece which bear densely-whorled branches, but which, on the germination of the carpospore, give rise to a laxlyfilamentous, somewhat irregularly branched plant, from which the ordinary sexual plants arise at a later stage. This filamentous structure has been attributed to the genus Chantransia, which it greatly resembles, especially when, as is said to be the case in Batrachospermum, it bears similar monospores. The true Chantransia, however, bears its own sexual organs as well as monospores. To the specific identity of Haplospora globosa and Scaphospora speciosa, and of Cutleria multijida and Aglaozonia reptans, reference has already been made. Again, many Green Algae — some unicellular, like Sphcerella and Chlamydomonas; some colonial forms, like Volvox and Hormotila; some even filamentous forms, like Ulothrix and Stigeoclonium—are known to pass into a condition resembling that of a Palmella, and might escape identification on this account. It is, on the other hand, a danger in the opposite sense to conclude that all Chantransia species are stages in the life-cycle of other plants, and, similarly, that all irregular colonial forms, like Palmella, represent phases in the life of other Green Algae. Long ago, Kutzing went so far as to express the belief that the lower Algae were all capable of transformations into higher forms, even into moss-protonemata. Later writers have also thought that in all four groups of Algae transformations of a most far-reaching character occur. Thus Borzi finds that Protoderma viride passes through a series of changes so varied that at different times it presents the characters of twelve different genera. Chodat does not find so general a polymorphism, but nevertheless holds that Raphidium passes through stages represented by Protococcus, Characium, Dactylococcus, and Sciadium. Klebs has, however, recently canvassed the conclusions of both these investi-