Shut your eyes and picture a seed. Whether it’s the tiny black dot in the center of your watermelon or a bean kernel, each seed contains an embryo that will give rise to a new plant when given the right conditions.
Unlike ferns and liverworts, seeds allow plants to reproduce. The utility and beauty of seeds make them remarkable.
Definition
Seed is the smallest unit of a plant that can give rise to a new plant. It contains a developing embryo and food reserve in a protective outer covering called the seed coat.
Seeds are extremely versatile, capable of surviving extreme conditions that would kill any other plant, such as dryness or heat. Their ability to survive is an evolutionary adaptation that enables them to continue the propagation of plants and ensure the survival of species.
The embryo inside the seed consists of the precursor tissues for different parts of a plant. It grows to a certain size and then growth is halted by the mother plant. The seed has a hilum at the center of its integument and a micropyle at the top of it.
The seed may have one (Monocotyledonous seeds) or two cotyledons, the leaf-like structures that supply reserves of food for the embryo. The cotyledons are connected to the embryonic axis by the epicotyl, which can be smooth or feathery.
Structure
Seeds are surrounded by food reserves called endosperm and enclosed in an outer protective covering called the testa. They are the characteristic reproductive organ of flowering plants (angiosperms) and gymnosperms like conifers and cycads. Seeds perform a wide range of functions including multiplication, dormancy, germination, dispersal and survival.
Seed structure varies between dicots and monocots. Generally, the embryo is linear with one or two seed leaves or cotyledons and an axis below the cotyledons that gives rise to the root and a miniature shoot known as plumule or epicotyl.
The cotyledons contain the embryonal food material (endosperm) in some seeds and the pericarp or fruit wall contains it in others. When the hard seed coat opens to allow water to imbibe, the cotyledons swell and the embryo elongates to initiate growth. In some cases, the embryo may use all of the stored food and the cotyledons disappear from the seed. This is known as polyembryony and distinguishes monocotyledonous and dicotyledonous angiosperms.
Function
Seeds are the source of food for humans and other animals, including most cereals and legumes. They contain protein, starch and oil.
A seed’s ability to survive in harsh environments makes it the ultimate means of plant reproduction. Researchers are interested in the protective mechanisms that allow seeds to tolerate extremes of heat, cold, dryness and pressure.
During the period of physiological dormancy, the seed experiences repeated cycles of warm and cool temperatures which can cause physical damage to seeds. The activity of DNA repair enzymes (poly ADP ribose polymerases) is important for determining seed viability. In addition, stratification — the process of placing seeds in a cooler environment to break down dormancy — is critical for enabling germination. Seeds are also useful for storing the genetic material of plants. This function is important for crop diversity and the sustainable production of food. It is also critical for resumption of agricultural production after disasters. Effective seed systems provide farmers with timely access to quality seeds of preferred crop varieties.
Dispersal
Plants rely on seed dispersal to get their offspring to locations where they can germinate. Seeds are moved away from the parent plant by wind, animal, or water. The seeds of some plants, such as Witch Hazel and the oxalis, explode from their pods or fruits to launch themselves into the air as projectiles. Others, like the nutmeg tree (Myristica fragrans) seeds, are carried along by water.
Many trees enclose their seeds inside fleshy fruits that are attractive to hungry animals. When the animal eats the fruit, the seeds are ejected from the digestive tract or drop from the animal’s fur or feathers.
The seeds of some trees, such as the tucum palm, are released into the water where they are carried downstream by the current. This type of seed dispersal is called aquatic or hydrochory. The fossilized feces of the American black bear and the moa (“Coprosma”) from New Zealand show that these animals also help with dispersal by transporting seeds in their stomachs.
