Seed: Definition, Meaning, Form, Function, Dispersal, Germination

Seeds are the fundamental units of reproduction in flowering plants that are formed after the fertilisation of an ovule. They enclose the embryo, endosperm for nourishment, and a protective coat to make them self-sufficient starting points for new plants. It is an important topic in biology.

Seeds are also important to agriculture and food security because they form the basis for the growth of essential crops. Therefore, understanding what a seed is, its structure, types of seeds based on the number of cotyledons and methods of seed germination, seed formation and development, along with the concept of seed dormancy, is important to know seeds better.

This Story also Contains

1. What is Seeds?
2. Structure Of A Seed
3. Types Of Seeds
4. Seed Formation And Development
5. Seed Dormancy
6. MCQ on Seed
7. Recommended video on "Seed"

What is Seeds?

A seed is a well-developed, fertilised ovule that comprises an embryonic plant together with a nutrient-rich endosperm enclosed within a protective outer coat. Being the principal reproductive unit of flowering plants, it therefore provides for the propagation and perpetuation of the plant species. Seeds become of prime importance in the reproduction of plants since they harbour information that produces a new generation of plants. They allow for the distribution of plant species in different environments and therefore resultant survival and adaptation of species.

Seeds also provide food for both humans and animals and are quite important in agriculture and ecosystems. The process of seeding therefore commences with fertilization, whereby a sperm is brought into contact with the egg in the ovule, and the two then combine to form one cell called a zygote after fertilisation has occurred. The fertilized egg undergoes cell division and further differentiation to develop into an embryo. Tissue around it undergoes differentiation and develops into endosperm, which becomes food for the developing organism. Outer layers of the ovule develop into the seed coat. When mature, the seed goes into a dormant state that is ready to germinate if favourable conditions are met and give rise to a new plant.

Structure Of A Seed

Understanding the structure of a seed helps in revealing how the plants protect, nourish, and regenerate their next generation. A seed contains important parts like the seed coat, endosperm, and embryo. Each component plays a unique role. The structure of a seed is explained below-

Seed Coat

The seed coat is the outer covering of the seed, formed from the integuments of the ovule. This provides physical protection to the seed against pathogens and dehydrating agents. It could either be of considerable thickness and hard, or thin and soft, depending on the species.

Endosperm

The endosperm is a tissue produced inside the seed of most flowering plants after fertilization. It envelopes and nourishes the embryo by providing all the vital nutrients in the form of starches, oils, and proteins that become very important during seed development and germination.

Embryo

That young, developing plant inside the seed is called the embryo. It is made up of a radicle (embryonic root), hypocotyl (stem), cotyledons (seed leaves), and an epicotyl (shoot). The embryo will start to grow into the mature plant at germination.

Types Of Seeds

Seeds vary on the basis of the number of cotyledons and their ability of germination. Their classification explains how different seeds grow and survive in changing conditions. The types of seeds are classified below-

Based On The Number Of Cotyledons

Based on cotyledons, the seeds are divided as Dicotyledonous and Monocotyledonous Seeds :

• Monocotyledons (Monocots)

Monocots are seeds with a single cotyledon, or seed leaf. Examples include grasses, lilies, and orchids. These plants usually have parallel leaf veins, scattered vascular bundles, and fibrous root systems.

• Dicotyledons (Dicots)

Dicots are seeds with two cotyledons. Beans, roses, and oak trees are good examples. Net-like leaf veins, rings of vascular bundles, and taproot systems should be usual in these plants.

Based On Germination

Based on germination the seed types are:

• Orthodox Seeds

Orthodox seeds exhibit tolerance to desiccation, which allows them to be stored for very long periods at low humidity and temperature. They include most of the crop seeds, like wheat, rice, and maize, which may be held in seed banks.

• Recalcitrant Seeds

The recalcitrant seeds are the ones that show intolerance to drying and have to be kept moist to remain viable. Species whose seeds fall in this category include coconut, avocado, and many tropical trees. The seeds of such plants are pretty hard to store since they retain their viability only under quite special conditions.

Seed Formation And Development

Seed formation starts after fertilisation and leads to the development of a new plant. It includes the transformation of the ovule into a seed and its germination into a seedling. The seed formation and development are explained below-

Pollination And Fertilisation

Pollination may be defined as the transfer of pollen grains from the male anther to the female stigma of flowers. Fertilisation occurs with the work of the grain of pollen germinates on the stigma, finally merging male and female gametes to form a zygote.

Formation Of Seed From Ovule

After fertilisation, the ovule develops into a seed. The zygote develops into the embryo, the integuments of the ovule become the seed coat and the endosperm develops to provide nutrition to the embryo.

Types Of Germination

• Epigeal Germination

The cotyledons get pushed above the soil level by the elongation of hypocotyl in epigeal germination. It occurs in beans and sunflowers.

• Hypogeal Germination

In hypogeal germination, the cotyledons are underground, and in epicotyl elongation, this is often found in peas and maize.

Seed Dormancy

Seed dormancy refers to a condition wherein the seeds may not germinate even in a favourable environment. Dormancy ensures that the seeds germinate at the right time to survive or grow, thereby aiding in the perpetuation of a species.

Causes Of Seed Dormancy

It may come in the form of physical barriers, chemical inhibitors, immature embryos, or the combination of any conditions which hinder germination until certain requirements are attained.

Types Of Seed Dormancy

• Physical Dormancy

The cause of physical dormancy is because of a hard seed coat. The seed coat forms impermeable water and gases. It is broken by mechanical scarification or natural means.

• Physiological Dormancy

Internal factors such as hormonal imbalance act to prevent germination. Dormancy can be broken by environmental cues or hormone treatments.

Methods To Break Seed Dormancy

1. Scarification

In scarification, the seed coat is mechanically broken or softened by heat or chemicals, allowing water and gases to enter, and then germination begins.

2. Stratification

This is the process involving the exposure of seeds to cold or warm conditions to simulate the change in seasons. Seeds that are placed under cold conditions for an extended period are said to be cold-stratified. Warm stratification, however, indicates treating seeds under warm conditions. Such treatments break the physiological dormancy.

3. Chemical Treatments

Chemical treatments include variously available chemicals like gibberellins or potassium nitrate that break dormancy and stimulate the germination of seeds physiologically dormant.

MCQ on Seed

Question : An example of a seed with endosperm, perisperm, and caruncle is:

Option 1 - coffee

Option 2 - lily

Option 3 - castor

Option 4 - cotton

Solution - Castor plant seeds are endospermic and dicotyledonous with the presence of the endosperm, in which there is a true food storage tissue. As such, there is supplementary nourishment coming from this seed via perisperm. Moreover, a specialized structure known as the caruncle acts as a savior regarding seed dispersal and seed germination.

Hence, the correct answer is option 3)castor.

Question: Monocotyledonous seeds are endospermic but ____ are non- endospermic

Option 1 - Wheat

Option 2 - Rice

Option 3 - Maize

Option 4 - Orchids

Solution - Monocotyledonous seeds - Monocotyledonous seeds are endospermic but some are non-endospermic (orchid). Monocotyledonous seeds are endospermic, storing food in the endosperm for the developing embryo, but some, like orchids, are non-endospermic. They have a single cotyledon, called the scutellum, which aids in nutrient absorption. The outer covering consists of the seed coat, which protects the embryo. Examples include seeds of grasses like wheat, maize, and rice.

Hence, the correct answer is option 4) Orchids

Question: Scutellum is ____ and is present in ____.

Option 1 - Embryo; dicot seed

Option 2 - Cotyledon; monocot seed

Option 3 - Cotyledon; dicot seed

Option 4 - Plumule; all seeds

Solution - The scutellum is cotyledon and is present in monocot seeds. The scutellum is a single, shield-shaped cotyledon present in monocot seeds, such as those of grasses. It plays a key role in the absorption of nutrients from the endosperm during seed germination. The scutellum is closely associated with the embryonic axis and facilitates the transfer of nutrients to the developing embryo. This adaptation is crucial for the growth and establishment of monocot seedlings.

Hence, the correct answer is option 2) Cotyledon; monocot seed.

Question: Seed is

Option 1 - Fertilized embryo

Option 2 - Fertilized ovary

Option 3 - Fertilized fruit

Option 4 - Fertilized Ovule

Solution - The Fertilized Ovule is the seed. Ovules convert into seeds. The fertilized ovule develops into a seed following the process of fertilization in plants. The zygote within the ovule grows into an embryo, while the ovule's integuments form the seed coat. The seed contains stored nutrients in the form of endosperm or cotyledons, supporting the growth of the embryo during germination. The ovary surrounding the ovule matures into a fruit, protecting the seed and aiding in its dispersal. Seeds are crucial for reproduction and propagation in plants, ensuring the survival of the species.

Hence, the correct answer is option 4) Fertilized Ovule.

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