Biology : The Time of Ancient Life : Part Six
Vascular and Nonvascular Plants
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 equal branches) appears a primitive or ancestral trait in vascular plants. Some branches terminated in sporangia that produced a single size of spore.
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.
|Left, Reconstruction of Baragwanathia longifolia, from the middle Silurian of Australia. Right, Baragwanathia longifolia, an Australian lycopod from Victoria and other localities. This plant has been controversial for some time because of its potential age as upper Silurian, predating or corresponding in age with much more primitive Cooksonia-like vascular plants. In the photograph above you can see the relatively stout axis thickly clothed by numerous helically arranged leaves. Scale is in mm.
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