The angiosperms of the plant kingdom are divided into two classes. These classes are divided on the basis of cotyledons. These cotyledons are also known as embryonic leaves or cotyledon seed leaf. The two types are dicotyledons and monocotyledons. Commonly we call them dicots and monocots. As the name suggests, the dicots have two cotyledons and the monocots have only one cotyledon. Pea and rose are the examples of dicotyledon plants and banana and grass are the examples of monocotyledon plants. The cotyledons are mainly formed when the seed develops. After the fertilization process, the ovules start to develop into seeds. Seed coat and embryo helping in making up the structure of the seed. Two or one cotyledons along with the embryonal axis help in making the embryo.
They have an outer covering that is called a seed coat. It is protective in nature. It is developed from the integuments of the ovule. Testa and tegmen are the two layers that are present on the seed coat. Testa is the outer layer and tegmen is the inner layer. There is a scar that is present on the seed coat and this scar is known as hilum. It helps in attaching the seeds to the fruit that is developing. The micropyle is a small pore that is present above the hilum. The embryo is the main part of the seed and this makes up the seed. In dicotyledons, the embryo is made up of an embryonal axis and two cotyledons. The cotyledon is fleshy in nature. There is an ample amount of reserve food material that is present in the cotyledons. On the two ends of the cotyledons, we have a plumule and a radicle. The plumule develops into a future shoot and the radicle develops into a future root. As a result of double fertilization, a structure that is known as endosperm is also present. It helps in storing the food and also provides nutrition to the developing embryo. Sometimes it is fully consumed in the process and sometimes it is not fully consumed. The seeds in which it is fully consumed are known as non-endospermic seeds and the seeds in which it is not fully consumed, it is known as endospermic seeds.
Maize is an example of a monocotyledon seed. It is cereal. The nature of seed coats in maize is membranous and this seed coat is fused with the fruit wall. Endosperm and embryo are the two structures that help in covering the seed. The endosperm in monocots is bulky in nature and it helps in storing the food. So from this, we can say that the monocots are endospermic in nature. But there is an exception to this case that is in orchids. In orchids, the seeds are non-endospermic which means that the whole of the endosperm is consumed at the time of development. The aleurone layer is a protective covering or layer that is present on the outside. This is just an outer covering of the endosperm and the function of this is to separate the embryo. On one end of the endosperm, we have the embryo that is present in the groove. As the monocot have only one cotyledon so the name of their cotyledon is scutellum. The cotyledon seed leaf is large and shield-shaped. This structure also has a plumule and a radicle. Coleoptile and coleorhiza are the two sheaths in which the plumule and radicle are enclosed. The coleoptile develops into future shoots and coleorhiza develops into future roots.
There are some seeds in which the nucellus is not fully consumed and these types of seeds are known as perispermic seeds. And this gives a name to the nucellus that is perisperm. These types of seeds are found in black pepper.
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Dormancy and Seed Germination
Dormancy is referred to a state when the embryo is inactive. This means that during this time the seeds are not able to germinate. This is achieved by decreasing the moisture content. The moisture content is reached to an amount of around 10-15%. They undergo such a phase so as to fight away the environmental conditions that are stressful for them. Whereas germination is regarded as the property when the seeds germinate when there are favourable conditions for their growth. This can be governed by the amount of moisture, oxygen, and a suitable temperature. These two characteristics help in the storage of seeds for a longer duration in agriculture. They can be used further and for many years.
Advantages of Seed in Angiosperms
We will learn about the advantages of seeds in angiosperms. They are:
For a dispersal to a new habitat, the seed has better adaptive strategies.
For the nourishment of young seedlings, there are sufficient food reserves present.
The hard seed coat serves the purpose of protection to the young embryo
As a result of sexual reproduction, new genetic recombination is produced.
Two cotyledon seed leaves are present.
There is a time when the seed attains the power of germination and this is known as seed viability. In oxalis, the seeds lose their ability to germinate within a few months. But, there are some seeds that have the ability to germinate even after hundreds of years. Lupine is the oldest of this type of seed and it was excavated from Arctic Tundra. This seed was dormant for a period of around 10000 years. A similar type of seed known as Phoenix dactylifera. It is the seed of a date palm and it is a 2000 years old viable seed.