Seeds - Structure and Germination

What is a Seed?

• Before  we talk about the germination  of  seeds it will  be appropriate to refresh  your knowledge of what three terms FRUIT, SEED, and GRAIN actually are:
  • Fruit is the enlarged ripened version of the ovarian wall forming the cell wall enclosing the seed.
  • The fruit protects the seed and helps in dispersal.
  • The seed is the ripened ovule.
  • It contains an embryo that develops into a new plant. The seed coat protects the embryo from mechanical damage.
  • Grain as found in maize, wheat, etc, is actually the fruit in which the fruit wall and the seed coat are fused together to form a protective layer.

More About Seed:

• It is a mature ovule after fertilization.
  • It contains a tiny living plant the embryo (developed from the fused sperm nucleus and the egg nucleus).
  • The embryo remains in an inactive(dormant) state until exposed to favorable conditions when it germinates.
    • The seed also contains food material for the nourishment of the embryo during germination.
    • The embryo can withstand unfavorable conditions of temperature, drought, etc. (Some seeds are known to remain dormant even up to 100 years or more).

Types of Seeds:

• Broadly the seeds are of two kinds i.e. monocotyledonous and dicotyledonous.
  • Monocotyledonous seeds contain one cotyledon (seed leaf)
  • Example: maize, grasses.
  • Dicotyledonous seeds contain two cotyledons.
  • Examples: peas, gram, and beans.
• Seeds vary in size.
  • Some are so small that they are barely visible to the naked eye.
  • Examples: poppy seeds, and orchid seeds.
  • Some are quite large as in watermelon and pumpkin or even in mango (the stone).
  • The largest seeds are those of coconut and double coconut.
  • The size, shape, and structure of seeds of different plants vary considerably but the basic structure of most seeds is the same
• On the basis of endosperm, seeds are classified as:
  • Albuminous (endospermic) cotyledons are thin and membranous and endosperm persists.
  • Examples: Dicot albuminous seeds: poppy, custard apple. Monocot albuminous seeds: cereals, millets, palm.
  • Exalbuminous (non-endospermic) - In such seeds, the cotyledon stores food and becomes thick and fleshy.
  • Examples: Dicot exalbuminous seeds - Gram, pea, mango, mustard, and Monocot exalbuminous seeds - Vallisneria, orchids, amorphophallus.

The Bean Seed:

• There are a number of different kinds of beans such as broad bean, lima bean, french bean, etc., but the general structure of their seeds is the same.
  • Most are kidney-shaped with a convex and a concave side.
• The seed coat consists of the testa the outermost hard brownish covering.
  • It protects the delicate inner parts of the seed from injury and from the attack of bacteria, fungi, and insects tegmen is a thin inner layer lying next to the testa, and this also is protective.
• Hilum is a distinct whitish oval sear on the concave side of the seed.
  • It represents the spot where the ovule (now the seed) was attached to the ovary wall through the placenta.
• A tiny pore micropyle is situated close to the hilum.
  • It marks the opening through which the pollen tube entered the ovule.
  • Micropyle serves two functions:
  • When soaked in water the seeds absorb water mainly through this micropyle and make it available to the embryo for germination.
  • It provides for the diffusion of respiratory gases for the growing embryo.
• Below the seed coat are two thick cotyledons which contain food for the embryo and protect it.
  • On carefully separating the two cotyledons the tiny embryo can easily be seen attached to one of the cotyledons.
  • The embryo consists of two parts-the radicle which later forms the root and the plumule which later forms the shoot.
• The plumule consists of a short stem with a pair of tiny leaves and a growing point between them.

The Maize Grain:

• The maize grain is actually a one-seeded fruit in which the fruit wall and the seed coat are fused together to form a protective layer.
  • Therefore, we call such fruit grain.
  • On one side of the grain occurs a small light-colored oval area which marks the location of the embryo inside.
  • The remaining major part of the grain contains a large endosperm which is rich in starch.
    • The endosperm and the embryonic part are separated from each other by a thin epithelial layer.
    • The outermost layer of the endosperm is rich in protein and is called the aleurone layer.
    • The embryo consists of a single cotyledon here called scutellum, a radicle, and a plumule.
    • The radicle is towards the pointed end and it is enclosed in a protective sheath, the coleorhiza.
    • The plumule is towards the upper broader side of the embryonic region and is enclosed in a protective sheath, the coleoptile.

Germination:

• The seed contains a dormant embryo. In a dry seed, the embryo is inactive. It is said to be in a state of dormancy (a period of rest).
  • Outwardly, it appears to be without life, but in fact, all the chemical activities of life are going on in it although they are very slow, and little food is utilized.
  • The dry seeds consume oxygen and give out carbon dioxide, both in extremely minute quantities, and they release some heat as well.
  • When placed under proper conditions the dormant embryo awakens, i.e. it becomes active and starts growing into a seedling.
• All the changes leading to the formation of a seedling are collectively called germination.
• Germination is the process of formation of seedlings developed from the embryo.
  • A fresh seed from a plant normally does not germinate even if the conditions for germination are favorable.
  • It must pass through a period of dormancy during which it undergoes physiological maturation.

Conditions Necessary for Germination:

• Water, suitable temperature, and air (oxygen) are necessary for germination.
  • Water: The seed obtains water from its environment, i.e. from the soil, in natural conditions.
  • The water is absorbed all over the surface but mainly through the micropyle.
  • Two main uses of water are:
    • The seed swells and consequently the seed- coat ruptures allowing the elongating radicle to come out and form the root system.
    • Water is necessary for chemical reactions and for the enzymes to act upon the food stored in the cotyledons or endosperm so that it may convert into a diffusable form dissolved and utilized by the growing embryo.
  • Suitable Temperatures: Both very low and very high temperatures are unsuitable for germination.
  • A very low temperature inhibits the growth of the embryo and a very high temperature destroys its delicate tissues.
    • A moderately warm temperature (25°C to 35OC) is usually favorable for germination and it is also called the optimum temperature.
    • Seeds of tropical plants often need a higher temperature for germination than those of temperate regions.
  • Oxygen: During germination, there is rapid cell division and cell growth for which energy is required.
  • This energy is available only by respiration (oxidation of food) and hence the need for oxygen (or air).

Some Experiments on Germination:

• Experiment to prove that water is necessary for germination.
  • Take two beakers and mark them A and B. In beaker A place some seeds of green gram (or pea, etc.) on wet cotton wool.
  • In beaker B place some similar seeds on dry cotton wool.
  • Keep both beakers in an ordinary room.
  • In a day or two, the seeds in beaker A will germinate but not in beaker B, showing that water is necessary for germination.
• Experiment to prove that a suitable temperature is necessary for germination.
  • Take two beakers and name them A and B.
  • Place some green gram seeds on wet cotton wool in each of the two beakers.
  • Keep beaker A in an ordinary room and beaker B in a refrigerator.
  • In a day or two, the seeds in beaker A will germinate, showing the importance of a suitable temperature for germination.
  • The seeds in beaker B may not show signs of germination or may germinate after several days though not to the extent the seeds in beaker A germinate.
• Experiment to prove that air (oxygen} is necessary for germination.
  • Take two conical flasks.
  • Name them A and B.
  • Spread wet cotton wool in each flask and place on it some soaked gram seeds.
  • Lower a small test tube containing alkaline pyrogallic acid, which absorbs oxygen, in flask B by means of a thread, taking care that not a single drop of the chemical falls on the seeds. or cotton wool.
  • Keep the tube hanging by fixing a cork on the mouth of the flask.
  • Arrange flask A in the same way, except that the test tube in this flask contains plain water.
  • Place the two flasks in an ordinary room.
  • The seeds in flask A will germinate showing the importance of oxygen for germination.
  • The seeds in flask B do not germinate because there is no oxygen (there may at the most be very slight germination due to anaerobic respiration in the absence of oxygen).
• The three-bean seeds experiment.
  • In this experiment, three mature air-dried bean seeds are taken and tied to a glass slide at three positions as shown in the figure. 
  • This slide is kept in a beaker containing water in a manner that the top seed is well above water, the middle one is just at the water level and the bottom one is deep in water.
  • The experiment set-up is left in a warm place for a few days result is as follows:-
  • The middle seed germinates. lt gets oxygen and water.
  • The top seed does not germinate at all. It gets oxygen but no water.
  • The bottom seed does not germinate or swim germinating after the emergence.

Types of Germination:

• The region of the axis between the point of attachment of cotyledons and the plumule is called epicotyl.
  • The region of the axis below the cotyledons is called the hypocotyl.
  • Both the epicotyl and hypocotyl of the seed never elongate together during germination.
  • It is either the epicotyl or the hypocotyl that elongates.
  • If the epicotyl elongates, the cotyledons remain underground (or on the ground if the seed is just on the ground) and the germination is then called hypogeal.
    • Examples: peas and gram.
  • If the hypocotyl elongates, the cotyledons are pushed above the ground and this type of germination is called epigeal.
    • Examples: castor, bean, etc.

Germination in some Common Seeds:

Pea Seed:

• The seed absorbs water and swells considerably.
• The testa softens and bursts.
  • The radicle emerges, grows downwards, and forms the root system.
  • The plumule grows upwards and forms the shoot of the seedling.
• In the earlier stages of development, the plumule is arched and thus protects the young shoot from injury during its emergence from the soil.
  • The cotyledons supply food till the seedling is able to exist independently.
  • Later they wither and shrivel up.
    • The cotyledons remain underground and germination is hypogeal

Bean Seed:

• The seed absorbs water and swells.
  • The radicle grows downwards to form the root system.
  • The arched hypocotyl grows to form an arc above the soil, it then straightens bringing the cotyledons above the soil.
• Germination is, therefore, epigeal.
• The cotyledons become the first green leaves and soon fall off after the foliage leaves grow.

Maize Grain:

• The grain imbibes water and swells considerably.
  • The radicle pierces through the protective root sheath (coleorhiza) and the fruit wall and grows downwards to form the root system, but it dies off soon.
  • The plumule pierces through its protective sheath, coleoptile, and grows straight upwards.
  • The two protective sheaths, coleorhiza, and coleoptile may be seen as a membranous covering on the axis of the seedling.
    • The cotyledon (scutellum) absorbs food from the endosperm till it is exhausted.
  • The hypocotyl does not elongate.
• Germination is hypogeal.