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The Siluria1 - Many scientists now...

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The Silurian Plants divide into two large groups: vascular plants that contain lignified conducting cells, and the nonvascular plants, which do not. Some Silurian plant fossils might be algae or nonvascular plants. Vascular plants developed during the Silurian period, 400 million years ago. The earliest vascular plants had no roots, leaves, fruits, or flowers. Cooksonia is a typical early vascular plant. It was less than 15 cm tall, with stems that dichotomously branched. Dichotomous branching (where the stem divides into two ewqual branches) appears a primitive or ancestral trait in vascular plants. Some branches terminated in sporangia that produced a single size of spore.
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Unformatted text preview: Many scientists now consider "Cooksonia" an evolutionary grade rather than a true monophyletic taxon. Their main argument is that not all stems of Cooksonia-type plants have vascular tissue. The evolutionary situation of a grade would have some members of the group having the trait, others not. The shapes of sporangia on various specimens of Cooksonia also vary considerably. Many of the early land plant fossils are not unequivocally those of vascular plants. The best such evidence is the presence of a trilete mark on spores produced by meiosis in a tetrahedral tetrad....
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