DEVELOPMENT OF NEW SEEDS

TOPIC: DEVELOPMENT OF NEW SEEDS

INTRODUCTION

Pollination is only the first step in the process of sexual reproduction in flowering plants. The development of new seeds in flowering plants begins after fertilization, when the male and female gametes fuse to form a zygote. This process involves the structure of the pollen grain and the structure of the mature ovule, which play vital roles in fertilization.

Structure of the Pollen Grain

The pollen grain is a haploid cell which represents the male gametophyte of a flowering plant. It is usually spherical or oval in shape and microscopic in size.

Parts of a pollen grain include:

1. Exine:
   • The outer tough, resistant wall that protects the pollen grain from desiccation and mechanical damage.
   • It is often sculptured and contains pores (called germ pores) through which the pollen tube emerges.
     
      
2. Intine:
   • The inner thin layer made of cellulose and pectin.
   • It is delicate and flexible.
     
      
3. Cytoplasm:
   • The living content of the pollen grain which contains food reserves and the male nuclei.
     
      
4. Nuclei:
   • A mature pollen grain contains two nuclei — a tube nucleus and a generative nucleus.
   • The tube nucleus controls the growth of the pollen tube, while the generative nucleus divides to form two male gamete nuclei.
     
      

Structure of the Mature Ovule

The ovule is the female reproductive structure in flowering plants found inside the ovary. It develops into the seed after fertilization.

Parts of a mature ovule include:

1. Integuments:
   • Protective layers that surround the ovule.
   • They leave a small opening at the apex called the micropyle.
     
      
2. Micropyle:
   • The small opening through which the pollen tube enters during fertilization.
     
      
3. Nucellus:
   • The tissue inside the integuments that provides nourishment to the developing embryo sac.
     
      
4. Embryo Sac:
   • The central part of the ovule where the female gametes (egg cell and polar nuclei) are located.
     It usually contains eight nuclei organized into:
     
      
     • One egg cell
     • Two synergids (beside the egg cell)
     • Three antipodal cells
     • Two polar nuclei (in the center)
       
        

FERTILIZATION IN FLOWERING PLANTS

Fertilization in flowering plants is called double fertilization because two fusions occur.

Process:

1. After pollination, the pollen grain lands on the stigma and absorbs nutrients, causing it to germinate.
2. A pollen tube grows from the pollen grain through the style toward the ovary, guided by chemical signals from the ovule.
3. The generative nucleus divides into two male gamete nuclei as the tube grows.
4. When the pollen tube reaches the micropyle, it enters the embryo sac.
5. One male gamete fuses with the egg cell to form a diploid zygote (2n) — this is true fertilization.
6. The other male gamete fuses with the two polar nuclei to form a triploid endosperm nucleus (3n) — this is triple fusion.
7. Thus, double fertilization results in the formation of the zygote (which develops into the embryo) and the endosperm (which provides nourishment).

After fertilization:

• The ovule develops into a seed.
• The ovary develops into a fruit.

Development of the Embryo

The zygote divides by mitosis to form many cells which differentiate and become organised into an embryo. The embryo is made up of:

• The plumule or embryonic shoot.
• The radicle or embryonic shoot.
• One or two cotyledons
• Hypocotyl and Epicotyl → stem regions

The radicle is the part of the embryo that develops into the root system of the new plant, and it is usually positioned close to the micropyle. The plumule is the embryonic shoot that gives rise to the stem and leaves of the young plant. The cotyledons, also known as seed leaves, serve mainly as food storage organs, providing nutrients to the developing embryo before germination. In some plants, especially monocotyledons like maize, the cotyledon also absorbs food from the surrounding endosperm. The number of cotyledons in a seed depends on the type of plant: dicotyledonous plants have two cotyledons (e.g., beans), while monocotyledonous plants have one (e.g., maize). The hypocotyl is the portion of the embryo found between the radicle and the cotyledons; it later forms the part of the stem below the cotyledons. Above the cotyledons is the epicotyl, which bears the plumule and later develops into the part of the stem above the seed leaves. These regions together form the basic structure of the embryo that will later grow into a seedling.

 Development of the Endosperm

During and after the development of the embryo, the endosperm also begins to form. The primary endosperm nucleus, which is triploid (3n), is produced when one of the male gamete nuclei fuses with the two polar nuclei during double fertilization. This nucleus divides repeatedly to form the endosperm tissue, a nutritive tissue that surrounds the developing embryo and provides it with food. The endosperm plays a vital role in nourishing the embryo during its early stages of development. In monocotyledonous plants such as maize and rice, the endosperm remains in the mature seed and continues to serve as a food reserve during germination. However, in dicotyledonous plants such as beans and groundnuts, the endosperm is usually absorbed by the growing cotyledons as the seed matures, leaving the cotyledons as the main food storage organs. The proper development of the endosperm is therefore essential for the healthy growth of the embryo and successful germination of the seed.

SEEDS

Definition:

A seed is a matured ovule that contains a young plant (embryo) capable of developing into a new plant under suitable conditions. It is the final product of fertilization in flowering plants and serves as the means by which plants reproduce and spread. 
The structure of a seed varies slightly between dicotyledonous and monocotyledonous plants, but the basic components remain the same.

Structure of a Seed
The seed generally has the following parts:

1. Seed Coat (Testa)
The seed coat, also known as the testa, is the outer protective covering of the seed. It develops from the integuments of the ovule after fertilization. The seed coat is usually tough and dry to protect the delicate inner parts of the seed — especially the embryo — from mechanical injury, water loss, and invasion by microorganisms. In some seeds, there are two layers: the outer testa and a thin inner layer called the tegmen. Together, they safeguard the seed until conditions are favorable for germination.

2. Hilum
The hilum is the scar or mark seen on the surface of a seed. It shows the point of attachment of the seed to the fruit wall through the funicle (seed stalk) when it was still inside the ovary. The hilum appears as a small oval or round spot, and it serves as a reminder of the connection through which food and water passed to the developing seed during its formation.

3. Micropyle
The micropyle is a tiny pore or small opening located close to the hilum on the seed coat. It is the same opening through which the pollen tube entered the ovule during fertilization. After fertilization, it remains as a minute hole on the seed surface. During germination, the micropyle allows water and air to enter the seed, which are essential for the resumption of growth. Sometimes, the radicle (young root) also emerges through the micropyle when the seed begins to germinate.

4. Embryo
The embryo is the young plant enclosed within the seed. It develops from the zygote formed after fertilization and represents the next generation of the plant. The embryo consists of several distinct parts:

• The radicle, which develops into the root system.
• The plumule, which grows into the shoot system (stem and leaves).
• The cotyledons (seed leaves), which either store food or absorb nutrients from the endosperm for the embryo’s development.

Around the radicle is the coleorhiza which is the sheath that protects the radicle while around the plumule is the coleoptile which is the structure that protects the plumule. In dicotyledonous seeds like beans, there are two cotyledons, while in monocotyledonous seeds like maize, there is only one (called the scutellum). The embryo remains dormant within the seed until conditions are favorable for germination, after which it grows into a new plant.

(Diagram: Essential Biology - Fig.37.5a and 37.5b pg. 511)


Structure of a Dicotyledonous Seed (e.g., Bean seed):

1. Seed Coat:
   Outer covering made up of two layers — testa (outer) and tegmen (inner).
   • Protects the seed from mechanical damage and infection.
     
      
2. Hilum:
   • The scar showing the point of attachment to the fruit wall.
     
      
3. Micropyle:
   • A tiny pore that allows the entry of water and air during germination.
     
      
4. Cotyledons (Two):
   • Store food for the developing embryo.
     
      
5. Embryo:
   • Consists of:
     • Radicle: The part that develops into the root.
     • Plumule: The part that develops into the shoot.
     • Hypocotyl and epicotyl: Connect the cotyledons with the radicle and plumule respectively.
       
        

Structure of a Monocotyledonous Seed (e.g., Maize grain):

1. Seed Coat and Fruit Wall (Pericarp):
   • Fused together forming a thin protective covering.
     
      
2. Endosperm:
   • Large food storage tissue.
     
      
3. Scutellum:
   • A single cotyledon that absorbs nutrients from the endosperm during germination.
     
      
4. Embryo:
   • Contains:
     • Radicle (covered by a sheath called coleorhiza)
     • Plumule (covered by coleoptile)

GERMINATION OF SEEDS

Definition:

Germination is the process by which involves the gradual development of the embryo of a seed into a seedling or young plant when suitable conditions are provided.

Types of Germination:
There are two types of germination: Epigeal and Hypogeal Germination

1. Epigeal Germination: The cotyledons are carried above the ground due to the elongation of the hypocotyl. It is usually associated with dicotyledonous plants. Example: Beans, melon., groundnut, etc.
   
    
2. Hypogeal Germination: The cotyledons remain below the ground due to the elongation of the epicotyl. It is usually associated with monocotyledonous plants. Example: Maize, millet, oil palm, etc. 
   
    

Process of Germination

1. Germination in Cowpea (Epigeal Germination)

In cowpea, the hypocotyl elongates and pushes the cotyledons above the soil surface. The process occurs in the following steps:

1. Imbibition (Water Absorption):
    The dry cowpea seed absorbs water through the micropyle, causing it to swell and the seed coat (testa) to split open.
2. Respiration:
    The hydrated seed resumes respiration, using stored food in the cotyledons to release energy needed for growth.
3. Growth of the Embryo:
    The radicle (embryonic root) emerges first and grows downward into the soil to form the root system, anchoring the seedling.
4. Elongation of the Hypocotyl:
    The hypocotyl grows rapidly and arches upward, pulling the cotyledons above the ground.
5. Emergence of Plumule:
    The plumule (embryonic shoot) develops between the cotyledons and grows upward to form the shoot.
6. Seedling Development:
    The cotyledons turn green and temporarily act as leaves, producing food by photosynthesis until the true leaves develop. Afterwards, the cotyledons wither and fall off.
   
    

2. Germination in Maize (Hypogeal Germination)

In maize, the epicotyl elongates while the cotyledon (called the scutellum) remains below the soil surface. The process takes place as follows:

1. Imbibition:
    The maize grain absorbs water through the micropyle, leading to swelling and activation of the embryo.
2. Respiration:
    The embryo begins to respire actively, utilizing stored food from the endosperm, which is digested by enzymes secreted from the scutellum.
3. Emergence of the Radicle:
    The radicle emerges first, protected by a sheath called the coleorhiza, and grows downward to form the primary root.
4. Growth of the Plumule:
    The plumule, protected by the coleoptile, grows upward through the soil and emerges above the surface to form the shoot.
5. Development of Seedling:
    The scutellum continues to supply nutrients to the growing embryo until the young maize seedling begins to photosynthesize and becomes independent.
   
    

CONDITIONS NECESSARY FOR GERMINATION
For a seed to germinate successfully, several essential conditions must be met. These factors help activate the embryo and support its growth into a seedling. They include:

1. Water or Moisture:
    Water is required to soften the seed coat, activate enzymes, and dissolve the stored food materials in the seed. The absorbed water (imbibition) allows the embryo to swell and begin active growth.
   
    
2. Air or Oxygen:
    Oxygen is needed for respiration, a process that releases the energy required for cell division and growth during germination. Without adequate oxygen, germination will be slow or may not occur at all.
   
    
3. Warmth or Suitable Temperature:
    A suitable temperature is necessary for enzyme activity and metabolic reactions. Too low or too high a temperature can prevent or delay germination. Most seeds germinate best between 25°C and 35°C.
   
    
4. Enzymes:
    Enzymes are biological catalysts that speed up the chemical reactions in the seed. They help convert the stored food in the cotyledons or endosperm into simple soluble forms that can be absorbed by the growing embryo.
   
    
5. Energy or Food:
    The embryo depends on stored food materials (starch, proteins, and fats) in the cotyledons or endosperm to obtain energy for growth until it can produce its own food through photosynthesis.
   
    
6. Viable Seeds:
    Only living or viable seeds can germinate. A viable seed contains a healthy embryo capable of growth. Dead or damaged seeds will not germinate even when all other conditions are met.

Seed Dormancy and How to Break It

Seed Dormancy
Seed dormancy is a state in which a viable seed fails to germinate even when all the necessary conditions for germination (water, air, and warmth) are provided. It is a natural mechanism that prevents seeds from germinating at the wrong time, such as during unfavorable environmental conditions (e.g., drought or cold). Dormancy ensures the survival of the species by delaying germination until conditions become suitable for growth.

Causes of Seed Dormancy

Seed dormancy can be caused by several factors, including:

1. Hard Seed Coat:
    Some seeds, such as beans and peas, have a tough seed coat (testa) that prevents the entry of water and air needed for germination.
   
    
2. Immature Embryo:
    In some seeds, the embryo is not fully developed at the time of dispersal and needs more time to mature before germination can occur.
   
    
3. Presence of Germination Inhibitors:
    Certain seeds contain chemical substances (like abscisic acid or tannins) that prevent germination until they are removed by washing or natural decay.
   
    
4. Lack of Light or Specific Temperature:
    Some seeds require light (e.g., lettuce) or a specific temperature (e.g., many temperate plants) to trigger germination.
   
    

Methods of Breaking Seed Dormancy

Dormancy can be broken artificially or naturally to enable seeds to germinate. Some common methods include:

1. Mechanical Scarification:
    Scratching, filing, or gently breaking the hard seed coat using sandpaper or a knife to allow water and air to penetrate. Example: Beans and other leguminous seeds.
   
    
2. Soaking in Water:
    Soaking seeds in warm water for several hours or days softens the seed coat and activates enzymes.
    Example: Maize, cowpea, and rice seeds.
   
    
3. Chemical Treatment:
    Treating seeds with mild acids (e.g., dilute sulphuric acid) or alkalis to weaken the hard seed coat.
    Example: Some forest tree seeds and leguminous plants.
   
    
4. Exposure to Light or Temperature Changes:
    Some seeds require exposure to light or alternating warm and cold temperatures (a process called stratification) to end dormancy.
    Example: Lettuce and temperate region seeds.
   
    
5. Leaching:
    Washing or soaking the seeds in running water to remove chemical inhibitors that prevent germination.
    Example: Some desert plant seeds.
   
    
6. After-Ripening:
    Storing freshly harvested seeds for a period to allow the embryo to mature before planting.
    Example: Apple and pear seeds.