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Vascular Plant Groups - members of the group having the...

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Vascular Plant Groups Vascular plants first developed during the Silurian Period , about 400 million years ago. The earliest vascular plants had no roots, leaves, fruits, or flowers, and reproduced by producing spores. Cooksonia , shown in Figure 8, 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. 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
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Unformatted text preview: members of the group having the trait, others not. The shapes of sporangia on various specimens of Cooksonia also vary considerably. Rhynia , shown in Figure 9, is another early vascular plant. Like Cooksonia , it lacked leaves and roots. One of the species formerly assigned to this genus, R. major , has since been reclassified as Aglaophyton major . Some paleobotanists consider A. major (Figure 10) a bryophyte, however, it does have a separate free-living sporophyte that is more prominent than the sporophyte, but appears to lack lignified conducting cells. The remaining species, R. gwynne-vaughanii is an undoubted vascular plant....
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