Seed formation, germination, structure, dormancy

 Seed

• Seed is a product of sexual reproduction and called fertilised ovule.

• It is developed after the double  fertilization from ovule or megasporangium.

• Seed has an embryo inside it that is either monocotyledon or dicotyledons. 

• A typical seed has a seed coat which is developed from the integuments  of the ovule, cotyledons and embryo axis.

Structure of seed

• The seed that is developed from bitegmic ovule,  has a double layer Seed coat .Outer layer is called testa and the inner layer is called tegmen.

• A typical seed has  mainly three structures . These are embryo, cotyledons and seed-coat.

• Both endosperm and embryo are the products of double fertilization, whereas the seed-coat develops from the integument of the ovule.

👌👌Remember -The seed habit is a significant advancement in the evolution of higher plants.

• In  addition to embryo, cotyledons and seed coat, seed  also has a micropyle in the form of pore that allows the entry of oxygen and water during seed germination.

• Cotyledons of embryo are simple structure. It is thick and get swollen dur to storage of food reserve as in legume plants.

• Hilum that connects ovule to ovary,may also present as out growth or scare inside the seed.

• In some seeds like Black pepper, beet, there is a remnant of nucellus called perisperm.

• In Acacia , Pithecellobium, a bright outgrowth is seen .This is called aril. When aril occurs as an outgrowth on a funicle. This is called strophioles.

• In Ricinus, when outgrowth is found around the  micropyle, it is called Carancle. In Litchi, arils are edible.

Albumunous and non albuminous seed

• Mature seed is of two type. It can be albumunous and non albumunous.

• Non albumunous seed has no residual of endosperm because it is completely consumed during the embryo development.

• Non albumunous seed are present in Pea and Groundnut.

• Albumunous seed has some remain of endosperm because it is not completely consumed during the embryo development.

• Albumunous seed are present in wheat, Barley, Maize, Castor.

Dormancy of seeds

• Seed germinates and forms a new plant. The seeds contain a reserve of food  in the form of cotyledons and endosperm.

• The seed coat is protective in nature which protects the embryo inside.

• As the seed mature, the water content is reduced and seed become dry. The metabolic activity of embryo inside the seed slow down and embryo enter a state of inactivity. This is called dormancy.

• After the dispersal of seed by the various agents like wind, water birds and animals, Seed may either germinate or attain the dormancy period or state of rest. 

• Dormancy is a  condition in which seeds are not germinated due to some unfavourable ecological conditions like  temperature, water, light etc. The dormancy period may vary from days to months and even years.

• For ex-  The oldest seed of Lupnius arcticus was excavated from Arctic Tundra.  This was  germinated after 10,000 years  of dormancy.

• Another example of sex of  Phoenix dactylifera that was also germinated after 2000 years. This seed was discovered at king Harold palace near the dead sea.

• Dehydration and dormancy of mature seed are important for storage of seed.

Artifecial breaking of seed dormancy

• Seed Dormancy is broken by the  natural and artificial methods.

• In the natural breaking of Seed dormancy, nature provide favourable  ecological condition such as moisture and temperature.

• The seed coat allows water to permeable in nature  and ruptures by the action of natural agents like microorganism, temperature, and  by the absorption  during digestion of birds and animals that feed on these seeds.

• Artificial breaking of Seed Dormancy include the action of hot water, rupturing of seed coats, Exposure to heat, cold or light etc.

• Hydraulic pressure is also applied  for 5 to 20 minutes  to make the weak  tough seed coats.

• Treatment of concentrated sulphuric acid is given to break the seed dormancy.

• Under the Scarification ,A hard seed coat becomes permeable to water or gases either by softening or cracking.

• This treatment can be either chemical or physical in nature.

• In the Stratification,  The seed is incubated  at the  low temperature over a moist layer before transferring to a temperature suitable for germination. 

• Incubation at 40-50 °C for a few hours is quite effective  to end the dormancy. For Example- , Rice seeds are treated with hot water at 40°C for at least 4 hours

• Plant growth regulators or other chemicals can be used in induced germination growth regulators.