The Importance of Seeds

What adaptations allow seed plants to reproduce without open water?

A characteristic shared by all seed plants is, as you might guess, the production of seeds. A seed is a plant embryo and a food supply, encased in a protective covering. The living plant within a seed is diploid and represents the early developmental stage of the sporophyte phase of the plant life cycle.

The First Seed Plants Fossils of seed-bearing plants exist from almost 360 million years ago. These fossils document several evolutionary stages in the development of the seed. Similarities in DNA sequences from modern plants provide evidence that today's seed plants are all descended from common ancestors.

The fossil record indicates that ancestors of seed plants evolved new adaptations that enabled them to survive in many environments on dry land. Unlike mosses and ferns, the gametes of seed plants do not need standing water for fertilization. Adaptations that allow seed plants to reproduce without standing water include a reproductive process that takes place in cones or flowers, the transfer of sperm by pollination, and the protection of embryos in seeds.

Cones and Flowers In seed plants, the male gametophytes and the female gametophytes grow and mature directly within the sporophyte. The gametophytes usually develop in reproductive structures known as cones or flowers. In fact, seed plants are divided into two groups on the basis of which of these structures they have. Nearly all gymnosperms (JIM noh spurmz) bear their seeds directly on the scales of cones. In contrast, flowering plants, or angiosperms (AN jee oh spurmz), bear their seeds in flowers inside a layer of tissue that protects the seed. Figure 22–15 compares the reproductive structures of gymnosperms and angiosperms.