ICSE Class 9 Biology Chapter 06 Seeds Structure and Germination

Unit 3: Plant Physiology

Seeds - Structure and Germination

Syllabus: Fruit and seed (definition and significance). Structure of dicot and monocot seeds. Germination of seeds, types, and conditions for seed germination. Structure and germination of Bean seed and Maize grain. Differences between monocot and dicot seeds. Differences between hypogeal and epigeal germination. Conditions for seed germination to be explained and supported by experiments.

6.1 What is a Seed?

Before we talk about the germination of seeds it will be appropriate to refresh your knowledge what the three terms FRUIT, SEED and GRAIN actually mean.

Fruit is the enlarged ripened ovary, the ovarian wall forming the fruit wall enclosing the seed. The fruit protects the seed and helps in seed dispersal. Examples: Mango, pea pod.

Seed is the ripened ovule. It contains embryo which develops into a new plant. The seed coat protects the embryo from mechanical damage. Example: Bean seed, peas.

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.

Teacher's Note

Understanding the difference between fruit and seed helps explain why we eat some fruits with seeds (like mangoes) and others we avoid seeds (like peas).

More About Seed

It is a mature ovule after fertilisation.

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 favourable conditions when it germinates.

The seed also contains food material for the nourishment of the embryo during germination.

The embryo can withstand unfavourable conditions of temperature, drought, etc. (Some seeds are known to remain dormant even up to 100 years or more).

6.2 Classification and Structure of Seeds

Types of Seed

Broadly the seeds are of two kinds - monocotyledonous and dicotyledonous.

Monocotyledonous seeds contain one cotyledon (seed-leaf) e.g. maize, grasses.

Dicotyledonous seeds contain two cotyledons e.g. pea, gram, bean.

Seeds vary in size.

Some are so small that they are barely visible to the naked eye e.g. poppy seeds, orchid seeds.

Some are quite large as in watermelon and pumpkin or even in mango (the stone).

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 same.

On the basis of endosperm, seeds are classified as:

(i) Albuminous (endospermic) cotyledons are thin and membranous and endosperm persists e.g. Dicot albuminous seeds - poppy, custard apple. Monocot albuminous seeds - cereals, millets, palm.

(ii) Exalbuminous (non-endospermic) - In such seeds, the cotyledon stores food and becomes thick and fleshy e.g. Dicot exalbuminous seeds - Gram, pea, mango, mustard and Monocot exalbuminous seeds - Vallisneria, orchids, amorphophallus.

1. The Bean Seed (Fig. 6.1)

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.

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 and tegmen (Fig. 6.1 D) is a thin inner layer lying next to the testa, and this also is protective.

Hilum is a distinct whitish oval scar on the concave side of the seed. It represents the spot where the ovule (now the seed) was attached to the ovary wall through placenta.

A tiny pore micropyle is situated close to the hilum. It marks the opening through which the pollen tube had entered the ovule. Micropyle serves two functions:

(1) When soaked in water the seeds absorb water mainly through this micropyle and make it available to the embryo for germination.

(2) 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.

Teacher's Note

Bean seeds are perfect for teaching seed structure because they are large enough to examine by hand, making botany tangible and exciting for students.

2. Maize Grain (Fig. 6.2)

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 a fruit as grain.

On one side of the grain occurs a small light-coloured 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 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.

The maize grain is monocotyledonous and endospermic. Some other examples of this type of grain are rice, wheat and oat.

Teacher's Note

Maize grains show how plants optimize energy storage - the starchy endosperm feeds both livestock and humans in countless food products.

Major Differences Between Bean Seed and Maize Grain

6.3 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 a seedling developed from the embryo.

A fresh seed from a plant normally does not germinate even if the conditions for germination are favourable. 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.

1. 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:

(i) The seed swells and consequently the seed-coat ruptures allowing the elongating radicle to come out and form the root system.

(ii) Water is necessary for chemical reaction 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.

2. Suitable temperature: 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 35-C) is usually favourable for germination and it is also called optimum temperature. Seeds of tropical plants often need a higher temperature for germination than those of the temperate regions.

3. 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).

Seeds sown very deep in soil fail to germinate. Two main reasons:

1. No proper supply of oxygen (for respiration)

2. Insufficient pushing force in the embryonic parts (hypocotyl or epicotyl) to break through the upper layers of soil.

Teacher's Note

Understanding seed germination explains why farmers plant seeds at specific depths and why moisture and air are critical for crop success.

6.4 Some Experiments on Germination

1. 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 the 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.

2. 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 cottonwool 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.

3. Experiment to prove that air (oxygen) is necessary for germination (Fig. 6.3)

Take two conical flasks. Name them A and B. Spread wet cottonwool 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 the 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 experimental set-up is left in a warm place for a few days and the result is as follows:

The middle seed germinates. It gets both oxygen and water.

The top seed does not germinate at all. It gets only oxygen but no water.

The bottom seed does not germinate or stops germinating after the emergence of a small radicle. It gets water but very little oxygen (from the air dissolved in water)

The experiment conclusively proves that water is essential for germination, but the other requirement of oxygen is not fully demonstrated.

4. The three-bean seeds experiment (Fig. 6.4)

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 seed does not germinate or stops germinating after the emergence of a small radicle. It gets water but very little oxygen (from the air dissolved in water)

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