A seed is a tiny package of genetic information that creates a new plant. It contains an embryo, stored food and a protective coat. Seed plants (gymnosperms and angiosperms) are now the dominant form of land plants.
Seeds can be bean-shaped (reniform) with lobed ends or square or oblong, broadest below the middle; they may also have wings for wind dispersal.
Definition
Seeds are the ripened ovules of flowering plants and similar gymnosperm (conifer) plants. The ovule is surrounded by the fleshy, nourishing tissue known as endosperm and protected by the hard outer seed coat.
A seed contains the embryo of a new plant and a supply of food until the embryo develops roots and leaves. Seeds are used for many foods, including cereal grains such as wheat, rice, barley and oats, legumes such as beans and peas, and spices like cumin, coriander, fennel, and caraway.
In sports, a player or team may be “seeded” to ensure that superior competitors will not meet in the early rounds. The term also applies to business ventures and research projects. The seed is the original idea that germinates into something bigger and more ambitious.
Parts of a seed
A seed contains an embryo that can develop into a new plant. It also has a supply of nutrients for the embryo and a hard shell to protect it. The outer shell is called a seed coat, and it protects the embryo from mechanical damage and parasites.
The inner part of a seed contains tissues that give rise to the roots, stems, and leaves of the new plant. These are known as the radicle and cotyledons. The radicle is a tiny embryonic root that emerges first during germination. The cotyledons store food in the form of starch and proteins.
The cotyledons may be thick and fleshy (as in wheat) or thin and hairy (as in cotton). In monocot seeds, the cotyledons provide nourishment to the embryo. Dicot seeds, such as soybeans, contain no endosperm.
Germination
All seeds have the potential to grow into a new plant once the right conditions are present. The seed embryo or miniature undeveloped plant and its food reserves are enclosed within one or more protective seed coats.
Germination happens when the seed’s outer coating ruptures and the embryo begins to grow into a sprout. The sprout then searches for light, growing toward it. Seeds that don’t find enough light are unable to support themselves and will eventually collapse.
Seeds from both flowering plants and gymnosperms require a period of time to break physiological dormancy. The method used for this purpose is called stratification and involves adding moisture to the seed to hydrate it, followed by a cold period to afterripen the seed. Stratification can also be done in the garden by using a crock pot or placing the seeds inside small jiffy pellets.
Dormancy
Dormancy is a protective mechanism that helps seeds survive adverse conditions. It can be induced by a variety of factors including light, temperature, soil moisture and chemical cues. It is also important during seed development and in the germination process.
Genetic factors influence the acquisition of dormancy in seeds and their ability to break it. For example, germination assays reveal that mutants that germinate more easily are less dormant than the wild type. However, these mutations do not necessarily represent genes that promote or inhibit germination, but rather those that affect dormancy and/or sensing of environmental stimuli.
Physical dormancy is broken by a period of stratification where seeds are incubated at low temperatures over a layer of moist soil. This breaks the inhibitory chemicals that are present in the seed coats.
Dispersal
Plants rely on several different methods for seed dispersal, which are often combined: gravity, wind, ballistic, water and animal-assisted. For example, seeds of some plants fall from the fruit or burst out when the pod dries up; others have hooks or barbs that get caught in animals’ fur and hitch rides farther away from the parent plant.
Water dispersal occurs in plants that grow near bodies of water such as oceans, lakes and ponds. These fruits are waterproof and float for long periods of time, which allows the seeds to be carried by currents over large distances. Examples include coconuts, swan plants and cottonwood trees.
Some seeds are enticingly packaged to encourage animal-assisted dispersal. These seeds have burrs, hooks or sticky hairs that attach to animals’ fur and feathers. Then, after the animals eat the fruit or seeds, they drop and deposit them in new locations far from the parent plant.
