A seed is a fundamental stage in the life cycle of flowering plants, serving as a protective package for the plant embryo. It is the product of a ripened ovule after fertilization, containing the genetic material necessary for a new plant to grow. Seeds consist of an embryo, a food supply, and a tough, outer protective layer called the seed coat. Flowering plants, known scientifically as angiosperms, are broadly classified into two major groups based on the internal structure of their seeds: monocotyledons and dicotyledons.
The Defining Feature: Two Cotyledons
The term “dicot” is a shortened form of dicotyledon, which literally translates to “two seed leaves.” This pair of embryonic leaves is the defining characteristic of this group of flowering plants. The cotyledons are formed during embryo development and are the first leaves to appear when the seed germinates.
The primary role of the cotyledons is to provide nourishment to the young plant until it can produce its first true leaves and begin photosynthesis. In many dicot seeds, such as beans and peas, the cotyledons become thick and fleshy because they absorb food from the endosperm, making them the primary storage organs. This stored food fuels the initial growth needed for the seedling to emerge from the soil.
Internal Structure and Anatomy
The dicot seed is designed for protection and dispersal. The outermost layer is the seed coat, or testa, which is a hard, protective covering that shields the embryo from damage and desiccation. Beneath the testa, a thinner inner layer known as the tegmen provides additional protection.
On the seed coat, a small scar called the hilum marks the former point of attachment to the fruit’s ovary wall. Near the hilum is the micropyle, a tiny pore that allows water to enter the seed, initiating germination. Inside the seed coat and cotyledons lies the embryo axis, which contains the rudimentary parts of the future plant. This axis features the radicle, which develops into the primary root system, and the plumule, which becomes the shoot, including the stem and leaves.
How Dicot Seeds Differ from Monocot Seeds
The difference in the number of seed leaves is the basis for the classification of flowering plants. Dicot seeds contain two cotyledons, while monocot seeds contain only one. This single difference correlates with four major distinctions in the mature plant’s anatomy.
Dicot leaves typically feature a branching, net-like venation pattern. Monocot leaves display parallel venation, with veins running side-by-side along the length of the leaf. The floral parts of dicots are usually arranged in multiples of four or five. Monocot flowers have parts arranged in multiples of three.
The root systems also diverge. Dicots develop a taproot system, characterized by a single main root that grows deep into the soil. Monocots develop a fibrous root system, consisting of a dense network of thin, branching roots. The vascular bundles, which transport water and nutrients, are arranged in a ring within the stem of dicots, while they are scattered throughout the stem in monocots.
Common Examples of Dicot Plants
The dicot group is incredibly diverse, encompassing the majority of flowering plants found across the world. They include many of the trees, flowers, and vegetables that are most familiar in daily life. Common garden vegetables such as beans, peas, and tomatoes are all dicots, as are many broad-leaved food crops like soybeans and lentils.
Many familiar trees are also classified as dicots, including oaks, maples, and fruit trees like apples and peaches. Showy ornamental plants, such as roses, sunflowers, and magnolias, are examples of dicots.