Two main structures provide evidence that the gymnosperms and angiosperms evolved from an ancestor that was not an ancestor to the other plants. Gymnosperms and angiosperms have both seeds and pollen. Since these evolutionarily derived trait are not found in other plants, they provides evidence that these groups all evolved from an ancestral species (the species you selected) which evolved these traits.
The plants with seeds and pollen have been extremely successful on land. To see why, consider the two traits:
As shown in the following figure, seeds contain, nourish, and protect the developing plant embryo. Seeds do such a good job of protecting the embryo from drying out, that they can survive for years until the appropriate conditions for development to arise.
As shown in the following figure, pollen is the tiny, two-celled haploid stage of the plant's life cycle. Because of its small size and light weight, the pollen can be dispersed by wind or animals from plant to plant; it can also survive in a dry environment. Once it reaches a receptive female gametophyte it develops sperm. Thus, unlike the plants that had evolved previously, seed plants are not dependent upon environmental moisture to carry the sperm to the eggs.
The University of California Museum of Palenotology (UCMP) currently has only a little information on the seed plants in general; if you are interested, or want to see if they've updated the exhibit, click here to visit the UCMP exhibit on seed plants.
Return to general information on the characteristics used to classify plants.