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

Introduction To 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

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

The types of seeds are classified below-

Based On The Number Of Cotyledons

Based on cotyledons, the seeds are divided as:

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 which 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

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 fertilization, 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

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.

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.

Chemical Treatments

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

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