Transition from plants producing only isospores,heterosporous plants with two kinds of spores, microspores and megaspores. Seeds first appeared in uppermost Devonian.

Evolution periods  

Four major periods of plant evolution is displayed in the fossil record and also by the diversity of modern representatives. Each period is characterized by the evolution of structures opening new opportunities on the land followed by an adaptive radiation.

The first period of plant evolution is defined as the origin from aquatic ancestors during the Ordovician period in the Paleozoic era, about 475 million years ago. This first terrestrial adaptation is marked by spores toughened by sporopollenin and jacketed gametangia that protect gametes and embryos.