Introduction to Seed Plants

Seed plants are the most diverse and ecologically dominant group of plants on Earth, with nearly 400,000 species. Scientists divide them into two major groups: gymnosperms and angiosperms.

Gymnosperms are nonflowering plants and include conifers, cycads, and ginkgoes. Angiosperms are flowering plants and make up more than 90 percent of all plant species, including flowering trees, grasses, and fruit-bearing plants.

Both gymnosperms and angiosperms share key features. Their life cycles are dominated by the sporophyte stage, which is the large, visible plant. They also produce highly reduced gametophytes.

The female gametophyte develops within an ovule, which protects and feeds it. The male gametophyte is the pollen grain, which is specialized for transport.

Plants produce pollen grains in specific structures—cones in gymnosperms and anthers in angiosperms.

Wind or animals carry the pollen to the female reproductive structure in a process called pollination. Fertilization can then take place.

After fertilization, the plant forms a seed. Each seed contains a diploid embryo, stored food, and a protective outer coat. These features help the embryo survive until conditions are right for growth.

Now, the differences between the two groups become clear. In gymnosperms, seeds are “naked” and sit exposed on the surface of cones or scales. In angiosperms, seeds are enclosed within fruits that develop from the ovary.

Fruits play an important role in seed dispersal. Animals may eat the fruit and later release the seeds in a new location. Some fruits float, fly, or attach to animals. These adaptations help seeds reach suitable places for germination.

When seeds reach favorable conditions, they germinate. At this stage, another classification becomes important: the number of cotyledons.

Cotyledons are embryonic leaves inside the seed. They are the first leaves to emerge during germination.

Gymnosperms usually have multiple cotyledons, often more than two. Most angiosperms have either one or two cotyledons. Based on this feature, scientists group them into monocots and eudicots.

Monocots include grasses, orchids, and corn, while eudicots include plants such as oaks and roses.

These two groups also differ in structure. Monocots usually have fibrous root systems, while eudicots have a taproot system. In stems, vascular tissue is scattered in monocots, while in eudicots, it forms a ring.

Leaves show parallel veins in monocots and net-like veins in eudicots. Flower parts usually appear in multiples of three in monocots and in multiples of four or five in eudicots.

Together, these structural and reproductive features help seed plants dominate life on land.