“What is the process of Seed Germination ?“, if you have the same question, stay tuned with this article because we are also going to cover about “How does a seed germinate ?” and “What are the stages of seed germination ?“.
The seeds which do not germinate even when provided with all the suitable conditions necessary for germination are called dormant seeds. This phenomenon is termed as dormancy.
But before knowing more about the process of seed germination, we should know “What is Seed Germination ?“.
So lets get started…
What is Seed Germination ?
The sum total of all the processes by which the dormant embryo of the seed grows out of the seed coats and establishes itself as an independent seedling is called germination.
According to Wikipedia, Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore.
When suitable conditions are available and the embryo has overcome dormancy, it becomes active and starts germinating. The process of germination is irreversible.
It was definition of germination of seed, lets discuss about the Conditions necessary for Germination of Seed.
Conditions necessary for germination
Seeds will germinate only if suitable conditions are available. The following favourable conditions are necessary for the germination of seeds :
- Availability of O2
- Moderate (favourable) temperature.
Apart from these conditions, light also plays an important role in seed germination.
1. Water – Dry seeds contain only 10% to 20% water. They are incapable of germination as long as sufficient quantity of water is not available.
Germination of seeds starts only after the rapid in- take of water through the micropyle and absorption through their surfaces. Water intake helps the seed to germinate in the following ways :
- It softens the seed coats and causes its rupture. The plumule and radicle emerge out as they grow.
- It activates protoplasm and enhances metabolic activities.
- It helps in the hydrolysis of stored food materials into simple and soluble state.
- It helps in the transportation of soluble food from storage tissue to the developing embryo.
2. Oxygen – During germination of seeds, metabolic activities accelerate and require energy. This energy is released from the stored food materials by the process of oxidation.
For this purpose, the seeds require oxygen. Because of oxidation of food, oxygen is consumed and CO, is produced. This exchange of O, and CO, takes place primarily through the micropyle.
3. Temperature – Temperature plays an important role in the germination of seeds. Seeds of different plants germinate at different optimum temperature (temperature at which the seeds grow most rapidly).
Generally seeds of tropical plants need higher temperature, whereas, that of temperate climates need low temperature for germination.
Process of seed germination
The seed contains a dormant embryo. When it is to germinate, rapid intake of water through the micropyle makes the seed large, swollen and the seed coat becomes soft and bursts open.
Absorption of water induces a number of physiological changes in the seed and enhances the respiratory activity. The embryo produces a large number of enzymes.
These enzymes hydrolyse the stored food material and make it available to the growing embryo. With the availability of food, cell division starts in the embryo.
Radicle and plumule grow in size and emerge out of the seed coat. The radicle is the first to come out of the seed coat. It forms the root system that grows into the soil.
The plumule after coming out of the seed coat forms shoot system. As mentioned above, the germination of seeds requires sufficient quantity of water, air and optimum temperature.
TYPES OF GERMINATION
The process of germination is categorized into two types depending upon the behaviour of cotyledons during germination i.e.
- Epigeal germination
- Hypogeal germination
In this type of germination, the cotyledons are lifted above the ground due to rapid elongation of hypocotyl. The cotyledons turn green and form the cotyledonary leaves.
- Under favourable conditions, the seed absorbs large quantities of water and swells.
- The seed coat softens and ruptures at the radicular end.
- The radicle pierces out of the seed coat, bends downwards and grows into the soil.
- The radicle forms the primary root. Lateral roots begin to emerge. Root hairs appear in the region where elongation has ceased. Water and salts absorbed by the root hairs are passed on to the seedling.
- Meanwhile, the hypocotyl elongates and arches upwards above the soil. The two cotyledons are within the ruptured seed coat.
- The hypocotyl straightens, pulling the cotyledons into the air. The ruptured seed coats may remain below the soil or are shed above the soil.
- The hypocotyl becomes completely erect. The two cotyledons diverge to form the flat, green cotyledonary leaves. The epicotyl elongates, bearing at its tip of the plumule.
- The young leaves unfold at the tip of the epicotyl. The cotyledons shrivel up and are shed.
- The primary root gives branches and persists all through the life of the plant. Such a root is called a tap root. The young leaves now prepare food for the plant.
When the cotyledons remain under the soil or just on the ground during germination of seed, it is called hypogeal germination.
It is because of the rapid growth of epicotyl that the plumule emerges out of the soil. It is found in the seeds of gram, pea, maize, mango, groundnut, etc.
Various stages during the hypogeal germination of maize and pea a monocotyledonous endospermic seed – a dicotyledonous non-endospermic seed – are summarized as follows :
Hypogeal germination in maize (Zea mays)
- The grain absorbs water and swells. The cotyledon absorbs food material stored in the endosperm.
- The radicle pierces through the protective root sheath (coleorhiza). It grows downwards into the soil.
- The radicle develops into the primary root with root hairs.
- The coleoptile elongates and pierces out of the grain in the form of a tube opposite in direction to the coleorhiza.
- A cluster of fibrous roots arise around the base of the primary root. These develop from the base of the stem. The first leaf of the plumule pierces out of the coleoptile which now forms a tube-like collar around its base.
- More leaves appear in succession. Primary root perishes giving rise to a fibrous root system.
Hypogeal germination in the seed of pea (Pisum sativum)
- The seed absorbs water and swells.
- Food reserves in the cotyledons become soluble by the action of enzymes and pass in the tips of the growing plumule and radicle.
- The seed coat ruptures, the radicle elongates, emerges from the ruptured seed coat and grows downwards into the soil.
- The epicotyl begins to elongate and forms a small loop. Root hairs appear in the region of the radicle where elongation has ceased.
- The epicotyl elongates rapidly. The plumule located at its tip becomes free from the cotyledons by the straightening of the epicotyl loop.
- Young leaves appear and prepare food for the plant. Seed coats and cotyledons remain below the soil.
- Primary root persists giving rise to a tap root system.
Special type of germination (Vivipary)
The seeds of the plants growing along the sea- shore or in saline marshes show a special type of germination called vivipary.
In this type of germination, the seed germinates inside the fruit while it is still attached to the parent plant. A club-shaped radicle emerges out of the fruit. The lower part of the radicle gets swollen.
Finally the seedling gets detached from the parent plant and gets embedded in the muddy soil below. The radicle develops lateral branches quickly to anchor the seedling.
This type of germination is found in Rhizophora, Ceriops, Avicennia, Heritiera, Sonneratia, etc.
Questions & Answers related to the process of Seed Germination
Que. What happen after Dormant Period ?
Ans. After the dormant period is over, the seeds germinate to establish an independent seedling, if provided with sufficient water, oxygen and optimum temperature.
Que. What is called Germination ? or Define Germination.
Ans. Germination is the sum total of all the processes which help in awakening the dormant embryo, its growth so as to establish an independent seedling.
Que. What conditions are necessary for the Germination of Seed ?
Ans. Germination of seeds requires sufficient quantity of water, oxygen and optimum temperature.
Que. What are the two types of Seed Germination ?
Ans. Germination of seeds is of two types i.e. epigeal and hypogeal germination.
Que. What is Epigeal Germination ? or Define Epigeal Germination.
Ans. In epigeal germination, cotyledons rise up the ground in the form of cotyledonary leaves due to rapid growth of hypocotyl. It is found in castor, bean, etc.
Que. What is Hypogeal Germination ? or Define Hypogeal Germination.
Ans. In hypogeal germination the cotyledons remain beneath the soil or just above the ground and the plumule rises due to rapid growth of the epicotyl. It is found in pea, gram, maize, etc.
Que. What is called Vivipary Germination ?
Ans. Seeds of plants growing along sea-shore or in saline marshes show a special type of germination called vivipary e.g. Rhizophora and Heritiera.
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Title : What is the process of Seed Germination ?
In most cases, the seeds are dry and incapable of germination when shed from the parent plant. In this condition, all the metabolic activities are taking place, but at the lowest rate just necessary for survival. A very small quantity of stored food is used.
This article mainly contains about the process of seed germination, the following topics are covered briefly in this article :
- What is Seed Germination ?
- Process of Seed Germination
- Condition necessary for seed germination
- Types of Seed Germination
- Special Type of Seed Germination