Seed-bearing plants include gymnosperms and angiosperms; they share several common characteristics, which are the ability to produce microspores, produce macrospores and conserve water. Seed-bearing plants are among the oldest plant species; the earliest plants and seeds arose over 360 million years ago. Seed-bearing ferns and gymnosperms, which include a number of conifers, arrived first, followed shortly by angiosperms, or flowering plants.
All seed-bearing plants have the ability to produce microspores and macrospores. Microspores enable the generation of pollen grains, which are sperm-bearing male gametophytes. The pollen grains then carry sperm cells to eggs, which enables fertilization to take place. Pollen grains allow sperm to transfer to eggs without water, and disseminate by drifting on air currents or being transported by pollinators. Seed-bearing plants also produce megaspores, which are essentially female reproductive organs. These spores develop inside ovules, and eventually produce seeds. Gymnosperms and angiosperms are relatively hardy species; they may survive on limited quantities of water by using water-conserving traits, which include thick cuticles on the leaves and wide vascular root systems. Despite sharing common traits, gymnosperms and angiosperms differ in appearance. Gymnosperms, according to The University of California, include four groups: conifers, cycads, ginkgos and gnetophytes. These species take the shape of trees and shrubs, while angiosperms are plants and flowers.