Seed is a miniature undeveloped plant containing an embryo and food reserves, usually surrounded by a hard shell or other covering. It “wakes up” and germinates when conditions are right for it to grow.
Seeds are an important food source for animals and humans. They are also used in plant breeding to create new varieties of plants with desirable characteristics.
What is a seed?
A seed is the fertilized, mature ovule of a flowering plant (angiosperm) or gymnosperm plants (conifers, cycads and ginkgoes). It contains a miniature undeveloped plant and food reserves within its protective outer covering, called a seed coat.
Seeds are one of the most diverse forms of life on Earth, with an enormous range of shapes and sizes and many mechanisms for dispersal. In fact, they are the dominant way for plants to reproduce.
A seed’s outer coating, referred to as its testa, may be rough or smooth. It contains an opening, the hilum, where the seed was attached to the mother plant (the ovary). The micropyle (a short ridge) that persists from this point is the only visible evidence that the embryo was once part of the ovule. It is now pressed against the nutrient tissue, or endosperm, inside the seed. It is in the contact with this material that the seeds of flowering plants and some other plants begin to grow.
What are the parts of a seed?
The main parts of a seed are the seed coat, the embryo, and the endosperm. The seed coat is the protective covering of the ovule, which is typically hard and thick. It has an outer layer called testa and an inner layer called tegmen.
The embryo is the underdeveloped tissue that will develop into a plant. In some seeds (e.g. corn and wheat) the embryo contains a single radical, while others (e.g. beans and lilies) contain two.
The cotyledons provide nourishment to the embryo during germination. They can resemble tiny leaves in some plants or be fleshy in others. They also store food in the form of starch and proteins. Some seeds lack an endosperm, which allows the cotyledons to serve as the food source for the embryo. This type of seed is referred to as non-endospermic or exalbuminous. Other seeds have an endosperm but no cotyledons, which is referred to as endospermic or albuminous.
How do seeds germinate?
Seeds contain genetic information that, in the right conditions, can grow into a new plant. It is important to note that seeds are not clones of their parent plants; like human children the offspring will be a combination of both parents.
In the natural environment seeds often enter a dormant state that can be triggered by a variety of stimuli such as rodents chewing on the fruit or the seeds rubbing against rocks, freezing and thawing of surface water, or passing through an animal’s digestive tract. Seeds also rely on other kinds of stimuli that allow water and oxygen to penetrate their hard, impermeable seed coat. This process is called scarification.
Once a seed is in the right environment it begins to grow, but only if it can find enough water to support its growth. The researchers in this study found that a protein called FLOE1 is able to sense moisture in the vicinity of the seed. It almost instantaneously assembles inside the seed to “test the waters” – it can act as a go or no-go signal, stopping germination if water availability is poor or allowing it to proceed if the conditions are favorable.
How do seeds grow?
Seeds store genetic information that has the potential, in the right conditions, to grow into a new plant. They are fertilized inside a flower by pollen from another plant’s male cells and the female cells of the same parent plant (an ovule). When pollen mixes with an ovule, it grows into a tiny plant embryo. The embryo is surrounded by food stored in the endosperm and by the cotyledons, or first leaf-like structures. All of this is enclosed within a hard outer shell called the seed coat.
Seeds come in all shapes and sizes. Some are large, such as conkers, while others, like celery seeds, are dust-like. They may also be smooth or rough-coated.
When a dry seed is given the right conditions, it will “wake up.” It absorbs water through its seed coat, gets bigger, and produces enzymes that ramp up metabolic activity. The radicle, or first root, then emerges from the embryo. Then, as it continues to grow, the seed will produce its own food through a process called photosynthesis.