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By the late Devonian period, more than 360 million years ago, Earth was home to a great variety of land plants, many of which we discussed in Chapter 27. The land plants shared the hot, humid terrestrial environment with insects, spiders, centipedes, and early tetrapods. These plants and animals evolved together, each acting as agents of natural selection on the other.

In the Devonian, a new innovation appeared when some plants developed extensively thickened woody stems. Among the first plants with this adaptation were seedless vascular plants called progymnosperms, all species of which are now extinct. The progymnosperms included many large trees.

Another innovation, the seed, arose in the seed plants. Seeds provide a secure and lasting structure that protects the dormant stage of the embryo. A plant embryo may safely wait within its seed (in some cases for many years, or even centuries) until conditions are right for germination.

The earliest fossil evidence of seed plants is found in late Devonian rocks. Like the progymnosperms, these now-extinct seed ferns were woody. They possessed fernlike foliage but had seeds attached to their leaves. By the end of the Permian, other groups of seed plants had become dominant (Figure 28.1). Today’s living seed plants fall into two major groups, the gymnosperms (such as pines and cycads) and the hugely diverse group known as the angiosperms (flowering plants).