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The volvocine line of evolutionary development culminatesin species of the genus Volvox. These colonies axe composed of a thousand or more cells, each one like Chlamydomonas; additionally, each one is connected to adjacent cells by cytoplasmic strands. This hollow sphere of cells, which contains a rather precise number of cells, is called a coenobium. The coenobium reproduces sexually when certain of its cells differentiate as slender, flagellated gametes and others develop into large, nonmotile bile eggs. Fertilization here is called oogamy because of the differentiation of the gametes. Specialists in the algae consider the volvocine line of evolution an evolutionary dead end. There are no rigorously demonstrated homologies between the coenobia, with their chlamydomonad cells, and other multicellular forms. In the last century, the father of phylogeny, Ernst Haeckel, seriously proposed that coenobia were the evolutionary antecedents of the hollow blastula of certain lower invertebrates. In other words, Haeckel derived the animals from these green algae. What Haeckel overlooked is that no colorless volvocines are known (they never show animal-like nutrition), chlamydomonad cells are plant cells and not animal cells, and the lower invertebrates, except for certain sponges, do not develop from hollow blastulae. This problem will be examined, but the conclusion of the experts in this field bears repeating, namely, volvocine evolution is an evolutionary dead end. The remaining two trends in the green algae are not dead ends. The siphonalian line of evolution has given rise to several evolutionary experiments. Those giving rise to plates of cells, such as seen in the genus Pediatrum, are probably going no further, evolutionarily speaking, but a form such as Vaucheria, when it becomes colorless as in Saprolegnia, is thought to be ancestral to certain water molds. That is, from it there evolved certain of the fungi. Vaucheria is coenocytic; it is one large multinucleate organism, since the filaments have no cross walls. Its highly specialized gametes no longer show a chlamydomonad-like structure. (The homologies between Vaucheria and Chlamydomonas, the plesiomorph of the green algae, can only be established by the serial criterion. This criterion establishes indirect similarities through the use of intermediate forms that here constitute the siphonolian line of evolution.)

The tetrasporalian line of evolution is thought to be ancestral to the green plants that invaded land and make up today's mosses, ferns, trees, shrubs, and grasses. At the peak oftetrasporalian evolution, both tissue formation and alternation of generations in the life cycle occur. Tissue formation is seen in the specialized mass of cells that make up the holdfast that attaches the plant to the substratum and in the rest of the plant, the thallus, that extends into the watery environment.