The seed ferns are an extinct group of plants known technically as the Pteridospermales. As indicated by their name, the seed ferns had leaves which were fernlike in appearance, and they reproduced by making seeds. Some seed ferns resembled tree ferns (family Cyatheaceae), a still-living group of tropical plants which are treelike in appearance but which reproduce by making spores. The seed ferns, however, were more prostrate in stature.
The seed ferns originated during the middle Devonian period, about 380 million years ago. They were dominant plants from the late Devonian to the Permian period, about 300 million years ago, but became extinct shortly thereafter.
Although seed ferns resembled the true ferns (order Polypodiates), there are two major differences between them. First, seed ferns reproduced by making seeds, whereas ferns reproduce by making spores. Second, the stem of seed ferns increased in girth through the life of the plant, due to cell division in a specialized outer cell layer in the stem known as the cambium. The cambium of seed ferns produced secondary xylem and phloem—cells specialized for water and food transport—much as the cambium of vascular seed plants do today.
Many botanists believe that seed ferns or a close relative were the first plants to reproduce by making seeds. The development of reproduction by seeds was an important evolutionary advance, because it meant that plants no longer had to rely on water as a dispersal agent for their sperm cells. Therefore, seed production enabled the seed ferns and their descendants to colonize relatively drier kinds of terrestrial habitats. The modern seed-producing plants are the evolutionary descendants of the seed ferns, and are the dominant plants in nearly all terrestrial ecosystems today.
The seed ferns did not have flowers, so they could be considered primitive gymnosperms. However, the seeds of seed ferns developed on fertile leaves, which were very similar to their sterile leaves, which lacked seeds. In this respect, the seed ferns were very different from modern gymnosperms, such as conifers and cycads, which bear their seeds in cones, which are highly specialized reproductive structures.
The stems and vascular systems of seed ferns had certain ultrastructural features similar to those of cycads, a small group of gymnosperms currently found in tropical and subtropical regions. In addition, the ultrastructure of the seed of seed ferns was similar to that of cycads. Thus, many botanists believe that the cycads are direct descendants of the seed ferns.
See also Plant breeding.
Peter A. Ensminger