The pteridosperms and the cordaites appeared first. These plants were common in the wet tropical and subtropical coal swamps that covered much of the central United States between 345 million and 225 million years ago. One of the best-known pteridosperms is Medullosa. Medullosa had an upright stem between 3 and 8 meters (10 and 26 feet) high. The lower portion of the stem was covered with adventitious roots (roots that develop from stems or leaves, rather than from other roots). A number of large compound leaves arose from the stem tip. Ovules and pollen organs occurred singly on the leaves and not in cones. Pteridosperm pollen organs consisted of a number of elongate euspo-rangia that were commonly fused to form a ring. The seeds of Medullosa were quite large. Some reached lengths of up to 11 centimeters. Unlike other pteridosperms, Medullosa had multiple vascular bundles in the stem. Other gymnosperms have only a single conductive strand in their stems.
The cordaites were derived from Archaeopteris-like progymnosperms. Some species of Cordaites were trees, others were shrubs, and some were similar to modern mangroves. Cor-daites was common in swamp, floodplain, and upland environments. Long strap-shaped leaves up to 1 meter in length occurred at its branch tips. It resembled modern mangroves in having stilt roots.
Cones developed between the upper surface of some leaves and the stem of cordaites. Four rows of bracts were borne on the cone axis. Above each bract was a dwarf shoot that terminated in either male or female reproductive structures. Swedish botanist
Rudolf Florin believed that the woody seed-bearing scale of modern pine could be derived from the dwarf shoot of cordaites through a series of extinct coniferalean intermediates. His interpretation has been adopted in many textbooks. It has also been shown that the conifers did not evolve directly from the cordaites, although both groups undoubtedly shared a common ancestor.
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