A treatise on the British freshwater algæ / by G. S. West.

  • George Stephen West
Date:
1904
Catalogue details

Licence: In copyright

Credit: A treatise on the British freshwater algæ / by G. S. West. Source: Wellcome Collection.

    44/402 page 18
    A Igce Many Algae exhibit an alternation of generations in their life- history. In those forms in which neither asexual nor sexual reproduction is known this alternation of generations is, of course, absent. Other Algae exhibit slight indications of an alternation of generations. Thus, in a large number of the Chlorophyceae, the sporophyte generation is represented by the zygospore. This zygospore can he described as a unicellular sporangium which often gives rise to two or four zoospores. Each zoospore, on coming to rest, germinates and produces the gametophyte generation. In the Mesocarpeae the isogamous gametes conjugate in a connecting- tube between the gametangia, and the zygospore immediately produces a rudimentary sporocarp consisting of a variable number of cells, one cell of which is the carpospore. This is the sporophyte generation. The carpospore, after a period of rest, germinates and gives rise to the new gametophyte generation. In the Rhodo- phyceae there is a well-marked alternation of generations. In the Algae the gametophyte is the important generation; in fact, the ‘plant’ is the gametophyte; but as one proceeds higher in the scale of vegetable life there is a great reduction of the gametophyte accompanied by a corresponding increase in the development of the sporophyte, until in the Phanerogams the sporo- phyte becomes the ‘plant’ and the gametophyte is parasitic on it. Polymorphism. Polymorphism occurs in most families of Algae. All those Algae which exhibit an alternation of generations are polymorphic, and some species appear to possess several different vegetative forms. It is very doubtful, however, if polymorphism occurs in Algae to the extraordinary extent advocaited by some authors. Sirodot has proved the occurrence of several types of structure in the genera Batrachospermum and Lemanea, and it is fairly evident that the plants at one time described under the genera Prasiola, Schizo- goninm and Hormidium are different vegetative forms of one genus. Polymorphism is well illustrated in Botrydium, and numerous striking instances could be mentioned of other Algae in which it occurs, notably in the common genus Pleurococcus. Hansgirg, and following in his footsteps Wolle, have endeavoured to prove (on insufficient evidence) the existence of extraordinary polymorphism in Algae, relegating most of the unicellular Algae as
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