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