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The green algae have sometimes been classified among the Metaphyta for the obvious reason that many of their member species are organized as highly integrated colonies or even as multicellular plants. We have treated them as protistans because of their obvious derivation from unicellular algae. By contrast, the brown algae (Phaeophycophyta) show no unicellular forms except for gametes, and in the red algae (Rhodophycophyta) there are only a few unicellular species.

Multicellular plants typify the red and brown algae. They are differentiated into the same general structures we encountered in the green algae, i.e., tissues specialized as holdfasts to anchor the plant to the substratum, and above that a buoyant thallus or vegetative structure. The thallus varies enormously in size and shape. The largest ones, found in the California kelps, are long blades of brown algae 50 m or more in length. Furthermore, the thallus can be organized variously into unbranched or branched structures and can be flattened blades or cylindrical stems. Often in these algae reproductive structures form another kind of tissue specialization with male and female gametes being produced in different parts of the plant. Finally, alternation of generation occurs. In the red algae there can be three different generations of plants before a life cycle is completed. This involves various combinations of haploid and diploid stages.

No motile cells have ever been found in the red algae, not even among their gametes. The sexual structures typically produce well-differentiated eggs and sperm--so well differentiated, in fact, that fertilization is referred to as oogamy. The male gamete is released into the watery environment of these plants and apparently, through random motion, comes into contact with and fertilizes the egg. The brown algae, however, have flagellated gametes and zoospores. Fertilization of gametes produces a zygote that germinates and develops into a sporophyte--the diploid phase of these plants. Meiosis occurs in the special cells that form zoospores, and these flagellated cells swim free and attach to the bottom where they then develop into haploid gametophytes.

There are various parallels between the organization of thethallus in the red and brown algae and the tetrasporalian line of evolution in the green algae. The result is an impressive array of multicellular aquatic plants. Quite naturally, the question arises as to which of them might be ancestral to the land plants. Or perhaps, the land plants are polyphyletic and have ancestors in all three algal phyla. Answers become clearer when we make certain other comparisons between these algae and the terrestrial plants. All these plants contain chlorophyll a among their photosynthetic pigments, but differ in terms of the other pigments, except for the Chlorophycophyta and the terrestrial plants. These latter two groups have the same photosynthetic pigments, food reserves, and cell wall components. This strongly suggests that the green algae were ancestral to these terrestrial plants. The red algae are the most different in terms of the characters cited. Note their photosynthetic pigments: these include phycoerythrin and phycocyanin, which occur only here and among the blue-green algae. This too has important evolutionary implications.