Advanced spore-bearing plants with true leaves and roots evolved during the Devonian: lycopods (club mosses), spheno-phytes (horsetails), pteridophytes (ferns), and progymnosperms (precursors to the seed-bearing gymnosperms). Some plants developed specialized woody tissue enabling them to attain the stature of trees.
Lycopods (lycophytes) formed a major part of the Devonian flora. Two distinct evolutionary lines developed from the lycophytes. One line, now extinct, evolved into the tall trees that dominated the Carboniferous coal swamps. The second group remained small and did not develop woody tissue. Living lycopods are generally low-lying plants with small, scale-like leaves. These small leaves probably represent modified stem outgrowths. Lycopod sporangia are positioned at leaf-branch intersections and are clustered in protective cones.
Horsetails (sphenophytes) have a jointed hollow stem and distinctive spiked leaves fused in whorls. Similar to lycopods, the sporangia are organized in terminal cones. Fossil horsetails tended to be much larger than modern forms. Some Carboniferous species grew up to 20 m in height.
Ferns (pteridophytes) are known from the Carboniferous and are the most common, living, spore-bearing plants. Sporangia are situated on the underside of large compound leaves, called fronds, formed of many leaflets. Each frond has a branched vein system. Such leaves likely evolved through the formation of webbing between branch tips. Most living ferns have leaves that are produced from a rhizome at ground level. Some fossil forms and tropical tree ferns have an upright trunk-like stem many meters tall.
The progymnosperms were the forerunners of the gymnosperms, seed-bearing plants. Superficially similar to tree ferns, these plants have woody trunks with a structure similar to some conifers. This gives them a vegetative structure similar to seed plants and reproductive mechanisms similar to ferns. Progymnosperms are only known from Devonian and Lower Carboniferous rocks.
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