Earliest Gymnosperms

The earliest recognized group of gymnospermous seed plants are members of the extinct division Pteridospermophyta, known as pteridosperms or seed ferns. These plants originated in the Devonian Period and were widespread by the Carboniferous. In habit, seed ferns resembled some progymnosperms in that they were small trees with fernlike leaves (the equivalent of a progymnospermous flattened branch) bearing seeds. Secondary vascular tissues were common in stems of seed ferns, though the wood was composed of thin-walled tracheids and abundant vascular rays, suggesting that stems were fleshy like those of cycads. Pteridosperm seeds were very similar to those of cycads and were often large, with a soft outer seed coat and a harder inner seed coat. Within a mature ovule was a massive female gametophyte with several archegonia. Some fossils suggest that the transport of the sperm through a pollen tube (siphonogamy) was in existence as far back as the Paleozoic. Pollen-bearing organs were variable among the pteridosperms; in many cases the microsporangia were elongated and fingerlike and were produced in clusters or were fused into compound organs.