A seed represents a plant’s fundamental reproductive unit, containing a young plant embryo. These compact packages of life enable plants to disperse and survive unfavorable conditions, waiting for the right moment to grow. The variety of seeds across different plant species is extensive, reflecting diverse evolutionary strategies for survival and propagation. This diversity includes a wide range of sizes, shapes, and internal structures, each adapted to its specific environment.
The Two Grand Divisions: Gymnosperm and Angiosperm Seeds
The plant kingdom’s seed-producing members are broadly categorized into two major groups: gymnosperms and angiosperms. Gymnosperms, which translates to “naked seeds,” produce seeds not enclosed within an ovary or fruit. Their seeds are typically exposed on cone scales or modified leaves. Familiar examples include conifers like pines, spruces, and firs, as well as cycads and the ginkgo tree.
Angiosperms, often called flowering plants, represent the largest and most diverse group of plants on Earth, encompassing approximately 80% of all known green plants. Their distinguishing characteristic is that their seeds develop within an enclosed structure, specifically an ovary, which matures into a fruit. This protective enclosure, whether a fleshy apple or a dry corn kernel, differentiates angiosperms from gymnosperms. Common angiosperm examples include nearly all fruits, vegetables, grains, and most nuts that humans consume, such as corn, beans, apples, and sunflowers.
Inside Angiosperms: Monocot and Dicot Seeds
Within the vast group of angiosperms, a further classification divides them into monocotyledonous (monocot) and dicotyledonous (dicot) plants, based on the number of embryonic leaves, or cotyledons, present in their seeds. Monocot seeds contain a single cotyledon, which is often specialized for nutrient absorption from the endosperm. Examples of monocots include major cereal grains like corn, wheat, and rice, along with onions and lilies.
Dicot seeds, in contrast, possess two cotyledons. These two seed leaves often become fleshy, serving as the primary storage sites for food reserves that nourish the developing seedling. Beans, peas, peanuts, oak trees, and sunflowers are all examples of dicot plants.
The Essential Components of a Seed and Their Variability
Despite their diverse appearances, most seeds share fundamental internal and external structures that enable them to protect and nourish the embryo. The outermost layer is the seed coat, which provides a protective barrier against environmental threats and physical damage. The thickness and texture of this coat vary considerably; it can be thin and membranous as in corn, or hard and woody as in a coconut.
Within the seed coat lies the embryo, which is the rudimentary plant itself, containing the beginnings of a root (radicle), stem (plumule), and the cotyledons. The cotyledons are embryonic leaves that serve various roles, including food storage or absorption of nutrients from other parts of the seed. In dicots like beans, the cotyledons are often large and store the majority of the food, while in monocots such as corn, the single cotyledon (scutellum) facilitates the transfer of nutrients from the endosperm.
The endosperm is a nutritive tissue that provides food for the developing embryo. In many monocots, the endosperm is prominent and persists in the mature seed as the primary food source. In many dicots, the endosperm is absorbed by the developing embryo, with the food reserves then stored directly in the cotyledons.