A seed is a reproductive organ of a plant that produces a new flowering stem. They are an important source of food, especially in animal and human diets.
A seed is made up of three basic parts: the embryo, the endosperm and the seed coat. Some seeds also have other appendages, such as raphe (ridge), wings and caruncles.
Origin
The seed is an important part of plant evolution. It is a fertilized ovule that contains an intact embryo, stored food and seed coat that can germinate into a new plant species.
It is also a major source of edible products, including cereals, nuts and legumes. It is also used for propagation in many plant species.
However, the origin of the seed remains a puzzle. There are various hypotheses based on morphological comparisons and fossil evidence.
In most monocotyledons (such as grasses and palms) and some (endospermic or albuminous) dicotyledons, the embryo is embedded in the endosperm (and nucellus). The remaining parts of the prothallus are mobilized later, during germination, to become the seed coat.
The seed trait is an evolutionary innovation that originated in plants 200 million years ago. While this trait is critical to human civilization, how it came to be remains an unresolved mystery.
Function
Seeds are a characteristic reproductive organ of both angiosperms (flowering plants) and gymnosperms. They are composed of a miniature plant embryo enclosed within a covering called the seed coat.
The seed coat is a complex organ of structure, chemical composition and metabolism that serves several functions during seed development. These include protecting the embryo and transmitting environmental cues to it.
A mature seed coat provides protection against pathogen penetration. It also enables the seed to sense and respond to its environment, as well as regulating nutrient flow toward the embryo and endosperm.
Seeds also protect the plant against transient herbivores and competition from other plants for light and nutrients. Many seeds are protected from germination by a process known as dormancy, which delays seed development for a period of time. This is useful because it allows seed dispersal at a later time when conditions are more favorable for growth.
Structure
Seeds are made of three basic parts: the seed coat, the endosperm, and the embryo. They are all necessary for the formation of a new plant, which uses the nutrients stored in them.
The seed coat is a layer of protective tissue that surrounds the endosperm and the embryo within a seed. It helps to protect the seed from damage and to retain moisture.
Many seeds have a scar on the outer surface that marks the location where the seed was attached to the ovary wall. This scar is called the hilum and it may also contain a small structure called a micropyle, which represents the end of the ovule’s micropyle.
Some seeds have additional structures such as an aril (a fleshy outgrowth from the funicle), a raphe, wings, or caruncles. Some seeds even have hairs, called trichomes, that help the seed grow.
Dispersal
The dispersal of seed is an important component of plant population dynamics. It is achieved through abiotic (wind and water) or biotic (fish, insects, reptiles and birds) mechanisms.
Unlike single-celled spores, seeds can transport nutrient materials to new sites. They also allow the spread of genetic variation, which is crucial for species survival and resilience.
Many species have evolved specialized structures or behaviors to enable them to achieve optimal seed dispersal. These include plumes for wind dispersal, barbs and attachment to animal bodies.
These structures and behaviors are often adapted to different species, habitats or life history strategies. They help plants to overcome obstacles such as competition, habitat loss and disturbances.
Seed dispersal is also a means for some species to escape environmental stress and to avoid predators and pathogens that are attracted to the parent plant. Moreover, some plants may select for dispersal because it reduces the risk of competition between parents and their offspring.
