Chapter 3:

Seeds: Structure And Germination

3.1 Structure of French bean seed

French bean seed is kidney-shaped. French bean seeds are the major source of proteins. Typically, a French bean seed consists of following parts.

Seed coat

The outer covering of a seed is known as a seed coat. In most seeds the seed coat consists of two layers such as:

i:- Testa

The outermost layer of the seed coat is hard and thick. This is called testa. It has different colours like white, yellow and grey black etc. It Depends on the type of a bean.

II:- Tegmen

The inner thin layer of seed coat is known as tegmen.

Cotyledons

Bean plant is a dicotyledonous plant as it contains two cotyledons. Cotyledons are thick and fleshy embryonic leaves. They store food and supply it to the developing embryo.

Embryo

Embryo is a tiny plant present inside the two cotyledons. Embryo consists of two parts:

i. The plumule which forms the new shoot.

ii. The radicle which forms the new root.

Hilum

Hilum is the site from where the seeds originally attached to the fruit.

Micropyle

There is a tiny pore called micropyle within the surface of testa. Water enters through it and starts the germination process.

The area above the Epicotyl cotyledons and below the plumule. It grows and becomes the stem of the plant.

Hypocotyl

Hypocotyl is the area between the cotyledons and the radicle. It grows and becomes the root.

3.2 Germination of Bean Seed

The process by which a seed begins to grow into a new plant under suitable conditions is called germination.

When bean seeds are sown in the soil, they swell by absorbing water through micropyle. The hard and tough seed coat now becomes soft The radicle starts growing first and makes the first root.

The root bursts through the testa and grows downwards. Small branches grow from the main root and hold the tiny plant in the soil. These roots absorb water from the soil.

After this the tiny plumule grows. It comes out from the soil and makes the shoot. The cotyledons also come out with plumule from the ground. This is called epigeal germination. The tiny root and shoot get stored food from cotyledons.

The plumule (shoot) continues to grow upwards. After sometime, new leaves grow on shoot. They are able to absorb sunlight. This young plant is called seedling. Seedling starts to make its own food by photosynthesis process. When all stored food of cotyledons is used, they shrink and die. In this way, germination of bean seed completes.

3.3 Structure of maize seed

Maize seed is a monocotyledonous seed as it contains one It is oval-shaped. Following are parts of maize seed.

Seed coat

Seed coat is the hard and tough outer covering.

Cotyledon

Maize seed contains single cotyledon. Its cotyledon does not store food for developing embryo.

Endosperm

In maize seed, food storage tissue for developing embryo is called endosperm) It is large and yellowish in colour.

Embryo

Maize seed is oval and flattened in shape. There is an embryo inside the seed. Its plumule is present at the broad end and develops into first shoot. Radiale lies near the pointed end and develops into first root.

Micropyle

Micropyle is a tiny pore through which water enters the seed. It is present on the pointed end of the maize seed.

3.4 Germination of Maize

Maize seed has one cotyledon, which does not store food. Food is stored in endosperm. Cotyledon uses the stored food of endosperm and gives it to radicle and plumule. When maize seed is sown in soil, it absorbs water through micropyle and swells. Its seed coat bursts.

Radicle comes out from seed coat and makes the first root. The Branches of root grow from first root. Then the plumule comes out of the soil and makes the first shoot. This shoot grows upwards and develops leaves. These leaves absorb sunlight and start to produce their own food by the process of photosynthesis. In this way, the germination of maize seed completes.

In this germination, the cotyledon remains inside soil and does not come out with plumule. This is called hypogeal germination.

3.5 Functions of Cotyledons

cotyledons have the following functions.

Cotyledons protect plumule.

In dicot seed, there are two cotyledons. They are large in size and have stored food for developing embryo.

In dicot seed, the cotyledons provide food to the seedling until it.starts to synthesizing own food thorough by photosynthesis.

In monocot seed, the size of cotyledon is very small and does not contain any stored food. Here the function of cotyledon is to use the stored food in the endosperm for embryo nourishment.

3.6 Conditions Necessary for Seed Germination

For seed germinations it is necessary that seeds are living and have adequate food storage. In addition, germination of seed is controlled by some conditions. Following are the some important conditions which are necessary

for the germination of seed. 1. Water or moisture

Air or oxygen
iTemperature
Water or moisture

Water Or Mositure

Water is essential for the germination of seed. The Seed absorbs water from the soil through micropyle and germinates. Due to this seed coat ruptures and the radicle and plumule come out. Later on after getting food and water, they grow into a complete plant.

ii:- Air or Oxygen

As embryo requires energy for its germination, this energy comes from respiration process. This process requires oxygen which is necessary for the germination of seed. As there are large spaces between the soil particles, so air present among the soil particles fulfills the need of oxygen.

iii:- Temperature

Different seeds require different temperatures for their germination. Most seeds germinate at temperature between 10°C to 35°C. Sunlight provides this necessary temperature to growing seed. Some seeds require low temperature before germination.