In botany, a seed is an undeveloped plant embryo and food reserve enclosed in a protective outer covering. More generally, the term “seed” refers to anything that can be sown.
Most seeds contain an embryo that grows into a seedling under optimum conditions. They also have endosperm, which is a storage reservoir for starch and oil.
What is a seed?
In botany, a seed is an undeveloped plant embryo and food reserve enclosed in a protective outer covering. They are the product of a fertilized ovule (the fruit of a flower) and, given appropriate growth conditions, become the new plant, which is called a seedling.
In angiosperms, or flowering plants, seeds develop from ovules contained in the ovary. These ovules are fertilized with sperm from pollen. The zygote forms a seed coat around the ovule and grows within the mother plant to a certain size before its development is halted.
The seed coat protects the seed from physical, mechanical, temperature-related and water damage. It contains the embryo, which germinates under ideal conditions and is nourished by endosperm. The cotyledon is the first leaf to develop within the seed.
During seed germination, the embryo inside the seed starts to develop and grow. It absorbs water and oxygen, and then begins to swell as the seed coat breaks open. Eventually a root or radicle emerges, followed by a shoot that contains the leaves and stem.
Seed germination requirements vary for different plant species, but generally include water, air, temperature and ultimately access to light. Some seeds require full sun for germination, while others germinate best in darkness.
Most seeds remain dormant or inactive until conditions are right for germination. This can occur when the seed is physically dormant (hard or thick seed coat), chemically dormant (abscisic acid or gibberellin levels), or a combination of both.
Endosperm forms the second stage in seed development and is the most important part of a seed. It is responsible for embryogenesis and stores food in the form of starch or other nutrients.
It is produced in both angiosperms and gymnosperms. It is formed during apomixis, which is the process whereby an asexual embryo develops without prior meiosis.
The development of the endosperm is dependent on the genomes of both parents. It also requires communication between the genes of the embryo and the surrounding seed coat.
There are two types of endosperm formation, the nuclear (or liquid) type where cell wall formation is delayed for a number of cell divisions and the cellular type where the walls are initiated instantly. Liquid endosperms are common in sweet corn and coconut water, while cellular types are found in seeds of wheat, barley and oat.
A cotyledon is the first leaf formed in the seed of a plant. It contains stored food that provides energy to the growing plant until it can produce its first true leaves and stem.
The number of cotyledons within a seed can vary from 8 to 20 or more, depending on the type of plant. Some cotyledons are only ephemeral, lasting days after germination; others persist for a year or more.
Many seeds germinate with a single cotyledon, which may remain below the ground (hypogeal) or emerge above it (epigeal). In plants like daylily, a cotyledon is usually hypogeal and remains below the soil until the true leaves are formed.
In a seed, there is also a structure called endosperm that helps to store nutrients for use by the developing embryo. It is a thick, fleshy structure with two ends, a plumule and a radicle. The plumule develops into a shoot and the radicle develops into a root.