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.