Seeds are the embryos of plants and contain food reserves and a protective seed coat. They are a way of reproduction for all flowering plants.
A seed’s ability to germinate depends on environmental conditions – water, temperature, sunlight and other factors. Germination usually occurs within a few days, but it can take up to a month or more.
Germination
A seed starts to grow when it undergoes a process called germination. This is when the embryo plant within it begins to grow, which leads to the formation of the roots and shoots that make up the new plant.
Germination requires the right temperature, moisture, air, and light conditions. Most seeds have optimal germination ranges (Table 1).
If the conditions are not right, the seed may die before germinating occurs. It is important to keep seeds moist, but not soggy.
When a seed is exposed to the right conditions, water and oxygen are absorbed through the outer shell of the seed. This causes the embryo cells to enlarge and the radicle to break through.
This is how the root grows down into the soil in search of water and nutrients to grow the plant. Once the radicle is anchored in the soil, the hypocotyl pushes upward and the cotyledons form. These cotyledons are the first leaves of the new plant, and they provide the seedling with its initial food supply until true leaves develop.
Parts of a Seed
In order for a seed to germinate, the right parts must work together. That means the correct amount of endosperm to feed the embryo, tiny beginnings of a root and shoot, and a protective seed coat to protect the embryo from the environment.
The first part is the embryo, which is a small, round, diploid (two-nuclear) plant that develops from the fertilized ovule. It consists of the epicotyl, hypocotyl, radicle, and one or two cotyledons in monocots and two or more in dicots and gymnosperms.
The next part is the protective coat, which is usually hard, thick, and resistant to water. It is made from the outer covering of the ovule, called integument. It consists of 2 layers: a testa, the thick outer layer, and tegmen, the more fragile inner layer.
Functions of a Seed
Seeds are the fruit of plants that have been fertilised with pollen. The ovules in a seed contain the embryo of the new plant, and a store of food that it will need until it has grown enough roots to take up its own supply of water.
They also have an outer layer called a seed coat that helps protect the embryo and endosperm from disease, insects or other damage. It also prevents water from entering the seed before it is time to germinate, so that the seed will stay dormant inside the coat until it germinates when conditions are right.
Once a seed has germinated, it will form a shoot and a root. It then takes up nutrients from the soil and sunlight. It then becomes an adult plant and produces new seeds.
Origins of Seeds
Seeds are a type of reproductive structure found in plants. They contain an embryo (EM-bri-o) or baby plant, a supply of nutrients for the embryo, and a tough outer layer called a seed coat.
They are often used as food, such as the seeds in a coconut, or as medicines like castor oil and tea tree oil. They also are used in breeding new varieties of plants.
The origin of seeds is a fascinating story that began in the late Devonian period, about 370 million years ago. It involved three important evolutionary trends: heterospory, integuments, and pollen receiving structures.
The emergence of the seed habit represents a major innovation in vascular plant evolution. It is the first time that reproductive tissues have been able to disperse and survive without the need for water. This change was marked by the development of specialized male microspores and female megaspores, as well as the evolution of integuments. In addition, the transition to water independence was made through the evolution of pollen receiving structures.
