Lepidodendron , the tallest Lycopodophyte, had an obconic stem, but the base was surrounded by cortical tissue that became thinner as the stem proper became thicker. As in Calamites , the kinds of cell present suggest that Lepidodendron did not have a particularly strong trunk. Moreover, the same swamps presumably provided good conditions for gametophyte development. Everything was just great, so far as these plants were concerned, but the climate changed. These plants had a stem structure very similar to modern gymnosperms.
Some fossils even show what look like annual growth rings.
Reproduction, however, was fernlike, i.e., spores were produced in sporangia. The fossil record suggests that most species were heterosporous, but some were homosporous. Any plants that had such a stem had an advantage over weaker stemmed plants in many habitats, although not in mesic lowland forests. Progymnosperms are also important because some among them moved from heterospory to seed production. This may seem a small step but, judging by the rapid diversification that took place and the current dominance of seed plants, it was one giant step for plants.