Seed plants are essential to life on Earth and are characterized by two key features: they have vascular tissue and reproduce using pollen and seeds. These adaptations allow them to live in a wide variety of environments and reproduce without needing water for fertilization.

Understanding the structures of seed plants helps explain how reproduction occurs. The pollen grain, which develops in the anther, contains the male gametophyte. It includes sperm cells, stored nutrients, and a protective outer coating that helps it survive during transport. The female structures, called ovules, develop within the ovary. Each ovule contains the female gametophyte, which produces the egg cell.

After fertilization, the ovule develops into a seed. Seeds have three main parts: the embryo, stored food, and a seed coat. The embryo is the young plant that develops from the fertilized egg, or zygote. It contains one or more cotyledons, or seed leaves, which often store food for the developing plant. The seed coat surrounds and protects the embryo, helping it survive until conditions are suitable for growth.

Once formed, the seed can germinate, which occurs when the embryo begins to grow and emerges from the seed under the right conditions, such as adequate water, oxygen, and temperature. For a plant to successfully grow and survive, seeds must be dispersed away from the parent plant. This reduces competition for resources and increases the chances of survival.

Seeds can be dispersed in several ways, including by wind, water, and animals. Some plants also use self-dispersal, in which seeds are forcibly ejected from the parent plant. These methods help spread seeds to new locations where they can grow into mature plants.

Seed plants are divided into two main groups: gymnosperms and angiosperms.  Gymnosperms are plants whose seeds are not enclosed in fruit, often described as “naked seeds.” Instead, their seeds are typically found on the surfaces of cones. Because they produce pollen, fertilization can occur without water, as pollen can be carried by wind directly to the female structures. The four main groups of gymnosperms are conifers, cycads, ginkgoes, and gnetophytes. Many gymnosperms, such as conifers, have needle-like or scale-like leaves and deep root systems that help them survive in a variety of environments.

In contrast, angiosperms, or flowering plants, are the most diverse and widespread group of seed plants. They reproduce sexually using flowers, which contain the reproductive structures. After fertilization, the ovary of the flower develops into a fruit, which surrounds and protects the seeds. The fruit also plays an important role in seed dispersal, helping seeds spread to new locations where they can grow.

Review:

1. What are the three main parts of a seed?
2. How are seeds dispersed?
3. Compare gymnosperms to angiosperms.