625

Seed plants have the remarkable ability to bring male and female gametes together when conditions are dry. How is this possible? Standing beneath a pine tree on a spring day, you will see the answer to this question all around you: tiny yellow grains that cover the ground and everything else close to the tree. These are pollen grains, and they are produced by all seed plants. In fact, in seed plants we encounter two new structures: pollen and seeds. Pollen is what liberates seed plants from the need to release swimming sperm into the environment, and seeds are multicellular structures adapted for dispersing offspring. The evolution of pollen and seeds required far-reaching changes to the seed plant life cycle.

There are two groups of seed plants living today: gymnosperms, which include conifers such as pine trees, and angiosperms, which are the flowering plants (Fig. 30.1). In this section, we examine aspects of the life cycle that are common to all seed plants and use pine, a gymnosperm, to illustrate how seed plant reproduction occurs without releasing swimming sperm into the environment. In section 30.3, we examine aspects of angiosperm reproduction, such as flowers and fruits, which distinguish their life cycle from other seed plants.