Section 3: Seed Plants
Seed plants are vital organisms to life on earth with two very important characteristics. They have vascular tissue, and they use pollen and seeds to reproduce. Examining the parts of seed plants helps us to understand how and where reproduction occurs. The pollen grain develops in the anther. It contains the male gametophyte, which consists of sperm cells, nutrients, and a protective outer coating. Ovules develop in the ovaries. They are the sporophytic structure, which contains the female gametophyte that produces the egg cell. The ovule forms the seed after fertilization. The seed has three main parts – the embryo, stored food, and a seed coat. The embryo is the young plant that develops from the zygote, or fertilized egg. In all seeds, the embryo has one or more seed leaves, or cotyledons, which sometimes store food. The seed coat protects the embryo. The seed then germinates. Germination occurs when the embryo begins to grow and pushes out of the seed. In order for a plant to thrive, seeds must be dispersed, ensuring a better chance for survival. Organisms, water, wind, and a self-ejection method can all aid in seed dispersal.
Seed plants are divided into two groups. Gymnosperms are plants that have seeds, but the seeds are not protected by fruit. The direct transfer of pollen allows fertilization to take place without gametes needing to swim through water. The four types of gymnosperms are conifers, cycads, ginkgoes, and entophytes. They have needle-like or scale-like leaves and deep growing root systems. Angiosperms, or flowering plants, are the most successful of seed plants. They reproduce sexually by means of a reproductive organ called the flower. After fertilization they often produce fruit, which is the mature, ripened ovary of a flower. The seed is protected by the fruit, and the wall of the fruit helps disperse the seeds.