Seeds: Know Seed Definition, Types of Seeds, Structure

Seeds are the hard structures inside fruits that we generally throw away. But what are seeds and what is their function? Seeds are the reproductive units of the plant. In fact, a seed is a plant itself in an embryo form enclosed in a protective outer covering. The term “seed” is also used to refer to anything that can be sown, e.g. “seed” of a potato is a tuber, not an actual seed, and when we say “seeds” of corn or sunflower “seeds”, what we refer to is the seed enclosed in a shell or husk.

The formation of the seed is part of the process of reproduction in seed plants. If proper care and a proper environment are given, seeds grow into new plants. Seed plants are now dominant among the biological varieties found on land, from forests to grasslands both in hot and cold climates. So let’s learn everything about seed in this article.

Seeds: Definition, Classification, Functions

“Seed is a ripened ovule that contains an embryonic plant with sufficient reserve food for the development of an embryo, the ovules after the fertilization process develops into seeds.”

Types of Seed

(a) Based on the number of cotyledons, seeds can be classified into two types,
1. Monocotyledonous seeds: These are the seeds with only one cotyledon.
Examples: Orchids, Alisma, corn, rice, etc.
2. Dicotyledonous seeds: These are the seeds that have two cotyledons.
Examples: Gram, pea, bean, mustard, mango, etc.

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(b) Based on the presence or absence of endosperm, seeds can be classified into-
1. Albuminous seeds (Endospermic): As the name suggests, these are the seeds with endosperm. In albuminous seeds, cotyledons are thin and membranous and endosperm persists and nourishes the seedling during its early development.
Examples: Castor, poppy, custard apple, etc.
2. Exalbuminous seeds (Non-endospermic): Seeds without endosperm are called non-endospermic or exalbuminous seeds. In exalbuminous seeds, food accumulates in the endosperm tissue at an early stage of development but it is utilized by the developing embryo and the mature seeds without endosperm. In such seeds, cotyledons store food and become thick and fleshy.
Examples: Gram, pea, bean, etc.

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Structure of Seed

The structure of seed differs in monocotyledonous and in dicotyledonous. Let’s know these structures in detail:

1. Structure of Monocotyledonous Seed

A monocotyledonous seed has one cotyledon. It has only one outer layering of the seed coat. The maize grain is an example of such a type of seed. It is a flat, almost triangular structure. Externally, it is differentiated into a large, upper area, which marks the position of the endosperm, and a small lower area that contains the embryo. These are separated by a prominent epithelial layer. Its detailed structure is as follows-

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Seed Coat: In cereals such as maize, the seed coat is membranous and generally fused with the fruit wall(pericarp), known as Hull.

Endosperm: The endosperm stores food. In general, monocot seeds are endospermic but some exceptions are there like orchids which are non-endospermic.

Aleurone layer: The outer covering of the endosperm detaches the embryo by a proteinous layer called the aleurone layer.

Embryo: The embryo is the small fleshy body seen after removing the seed coat, it is located in a groove at one end of the endosperm. The embryo consists of the following parts:
(a) Cotyledon: Monocots have only a single cotyledon. The scutellum is the equivalent structure in monocots, mainly in the grass family.
(b) Plumule: It represents the growing tip of the shoot along with embryonic leaves. It is covered with a sheath called coleoptile.
(c) Radicle: It is present at the basal part of a seed and is covered by a root cap. The protective undifferentiated sheath of the radicle is called coleorhiza.

2. Structure of a Dicotyledonous Seed

Dicotyledonous seeds have two cotyledons. Bean, peas, almonds, and cashews are some of the examples of dicot seeds. The detailed structure of dicot seed (e.g, bean) is as follows:
Seed Coat: Each seed is covered by a protective covering called a seed coat. It develops from the integuments of the ovule. The seed coat is differentiated into an outer testa and inner tegmen.
(a) Testa is the outer, thick, protective layer. It protects the seed from pathogens like bacteria, fungi, insects, etc.
(b) Tegman is an inner, thin, membranous covering that protects the inner part of the seed.

(c) Hilum: It is a scar on the seed coat through which the developing seed is attached to the fruit.

(d) Micropyle: This is a small pore present above the hilum. It is the entry for oxygen, water, etc., into the seed.

(e) Raphe: It is a ridge-like structure present around the median groove. It represents the portion of the stalk which remains fused with the test.

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Embryo: It has an embryonal axis and two cotyledons.
(a) Cotyledons: Dicots have two cotyledons that are food-laden, fleshy structures attached to the embryonal axis. They provide nourishment to the embryo in the early stages of development.
(b) Embryonal axis: A young miniature plant is present between the two cotyledons which is called the embryonal axis. It consists of the following parts:
1. Plumule: This part gives rise to shoot.
2. Radicle: This is the lower end of the embryo that gives rise to roots.

Seed Formation

Flowering plants undergo the process of fertilisation. After fertilisation, the ovule slowly matures into a seed whereas the ovary develops into a fruit.
These are the changes during the formation of seeds:
1. The fertilised egg (zygote) undergoes several divisions to form an embryo within the ovule.
2. The ovule develops a tough coat around it and is gradually converted into a seed containing the baby plant.
3. The ovary of the flower develops into a fruit with seeds inside it.
4. The other parts of the flower such as sepals, petals, stamen, stigma, and style dry up and fall off.
5. Now, we can see fruit at the place where we had a flower, fruits protect the seed. Some plants like mango, oranges, etc., have sweet, soft, juicy fruits whereas some like peanuts, almonds, etc., have dry and hard fruits.

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Seed Dispersal

Seed dispersal is the mechanism that helps in the transport of seeds to other places to ensure the germination and survival of at least a few into the adult plants. Seeds contain embryonic plants. Their dispersal is of high significance in the distribution of plants to many geographical areas. Seeds must be properly spaced to develop into healthy new plants and in order to avoid any competition for the basic needs of the baby plants. Thus, they have developed various mechanisms for their distribution to ensure that at least a few of them can meet favourable conditions for germination and proper growth.

Types of Seed Dispersal

There are several ways in which seeds can be dispersed from their parent plant, some of these are mentioned below:

Seed Germination

When the seed gets suitable conditions like water, air, and warmth, etc., it sprouts and a new plant grows out of the seed. The growth of a seed into a young plant or a seedling is called as germination of a seed. Seeds that are obtained from the plant are usually hard and dry. In this state, seeds remain alive but inactive for a long period. At this stage, seeds are called dormant.
1. Germination begins when the seed absorbs water, swells up, and bursts open through the seed coat.
2. The water absorbed by the seed helps the enzymes to function. It digests the stored food in cotyledons and makes it soluble.
3. This soluble food makes the radicle and plumule present in the seed develop further.
4. The radicle of the seed grows first to form the root that grows down into the soil and begins to absorb water and minerals from the soil.
5. After this, the plumule grows in an upwards direction to form the shoot. When the shoot grows above the ground, it develops green leaves.
6. The root and shoot continue to grow further. Green leaves of shoots begin to synthesize their own food in the presence of sunlight (photosynthesis).
7. Gradually and ultimately, the seedling develops into a new plant.

Significance of Seeds

Seed plays an important role not only for plant growth as well as for animals. There is a wide range of animals that depend on seeds for their nutrition(food). Some of the significance of seeds are as follows:
1. Complete Package: Seed is a complete unit for the endurance and growth of the new plant due to dormancy of embryo, presence of stored food, and protective seed coats.
2. Perennation: Due to the presence of tough impermeable seed coats and dormant embryos, seeds can pass through unfavourable conditions without any harm.
3. Dispersal:  Seeds are small and light, hence can easily be dispersed to long distances.
4. Delayed Germination: Seeds can be stored for months and even years. They will germinate only when provided with suitable conditions.
5. Food: Animals such as humans, birds, small insects feed on seeds of various plants.

Summary

Even though plants can reproduce through other methods, seeds serve as the most reliable method of reproduction. The most important feature of seeds is that they can remain in the dormant stage for a long time and can germinate when it meets necessary conditions. It stores food required for the growth of the embryo and provides protection to it. Seeds also help in the spread of species to a wide range of locations. Apart from plants, seeds are also essential for us in various ways, for example, cereals and legumes that we eat are used by a large population.

Frequently Asked Questions

The most frequently raised queries on seeds by students are answered here:

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