A seed is a self-contained biological package designed for plant reproduction. This structure contains an undeveloped plant called the embryo, a store of food for initial growth, and a protective seed coat. The development of the seed allowed plants to reproduce without relying on standing water, enabling them to colonize diverse terrestrial environments. Its ability to enter dormancy and survive harsh conditions makes the seed an extremely successful adaptation for species survival and dispersal.
Classification by Evolutionary Group: Gymnosperms and Angiosperms
The most fundamental division of seed types is based on how the parent plant protects them, separating them into two major evolutionary groups. Gymnosperms, meaning “naked seeds,” represent the older lineage of seed plants. Their seeds are not enclosed within an ovary; instead, they are openly exposed, typically developing on the scales of cones, as seen in examples like pines, firs, and cycads.
Angiosperms, or “enclosed seeds,” are the more recently evolved and dominant group, making up about 80% of all known green plants. Their defining characteristic is that the seed develops inside the ovary, which matures into a fruit. This fruit covering provides an extra layer of protection and often aids in dispersal. The presence of flowers and the resulting fruit distinguishes angiosperm and gymnosperm seeds.
Structural Division: Monocots and Dicots
Within Angiosperms, seeds are categorized based on the internal structure of their embryo, specifically the number of seed leaves, or cotyledons. A cotyledon is an embryonic leaf that stores or absorbs nutrients from the seed’s food supply. This nourishes the embryo until the true leaves can begin photosynthesis.
Monocotyledonous seeds, or monocots, possess a single cotyledon. In seeds like corn, rice, and wheat, the cotyledon is modified to absorb nutrients from a large, separate food storage tissue called the endosperm. Monocot plants often exhibit parallel veins in their leaves and have fibrous root systems.
Dicotyledonous seeds, or dicots, have two cotyledons. These seed leaves are often thick and fleshy because they store the majority of the food supply for the young plant. Consequently, the separate endosperm tissue may be reduced or absent. Examples include beans, peas, peanuts, and most flowering trees. Dicots are characterized by a net-like pattern of veins in their leaves and a prominent central taproot system.
Functional Diversity: Seed Dispersal Strategies
Beyond structural and evolutionary classification, seeds are functionally defined by the mechanism they employ to move away from the parent plant, known as dispersal. This movement reduces competition and allows colonization of new habitats. Seed morphology is often a direct adaptation to one of four main dispersal strategies:
- Anemochory, or wind dispersal, used by extremely light seeds or those with specialized structures to catch the air, such as winged maple fruits or dandelion seeds.
- Hydrochory, or water dispersal, utilized by buoyant and water-resistant seeds, such as the coconut, which can float across oceans.
- Zoochory, or dispersal by animals, which includes seeds with hooks that attach externally to fur, or seeds contained in fleshy fruits that are eaten and excreted internally.
- Autochory, or self-dispersal, which involves the plant actively projecting its seeds through a mechanical, often explosive, force, as seen in plants like the touch-me-not.