ALGAE - PHYLOGENY There can be little doubt that algae originated in the water and that they preceded in their evolution the more elaborate land plants. The different classes, as well as some prevalently flagellate groups dealt with elsewhere (see PROTOPHYTA), are distinguished by special pigments in the chromatophores and by the products of photosynthesis. Each class thus has its distinctive metabolism and probably represents the outcome of a separate attempt at the creation of a holophytic organism. There is little evidence of any real relationship between the diverse classes, and a common ancestry is in the light of our present knowledge improbable. In several classes the ciliated individuals and the motile reproductive cells are of a distinctive and rather uniform stamp, and this fact has led to the practically universal adoption of the view that these algal classes originated from motile unicellular organisms (Flagellates) that gradually lost their power of movement and gave rise to filaments, etc., in the way that has been above indi cated. In red and blue-green algae, where no motile stages exist, a like origin from unicellular forms is assumed, though these need not necessarily have ever had the capacity of movement. One may therefore picture the evolution of algae as having fol lowed a number of separate lines, starting each with its unicel lular ancestry and branching out in numerous special directions to develop as colonies, as palmelloid types, as filamentous forms with diverse elaborations, etc. It is unlikely that these principal categories have ever originated directly from one another. They would rather appear to represent as many different attempts at the building of a body from the unicellular ancestry. It is signifi cant that the same types of plant-body are repeated in different classes, since this indicates a parallelism in the evolution of the latter which has only recently been recognized. In some classes (e.g., Heterokontae) evolution has not progressed far, while in others (e.g., Phaeophyceae and Rhodophyceae where the simpler forms seem to be extinct) it has led to the production of highly specialized types in part of large size.
The biggest range in structure and reproduction is encountered in the green algae, although they have not attained to the com plexity or dimensions of the brown and red seaweeds. The absence of the more specialized forms in this very vigorous and adaptable class is possibly due to their having migrated to the land in the remote past, although others hold that the higher plants originated from a hypothetical group of large green seaweeds now altogether extinct, and at least as complex as the brown and red forms of the present day.