Modes of Reproduction and Apomixis

Modes of Reproduction in Plants

Reproduction refers to the biological process by which plants produce offspring. In plants, reproduction occurs via two primary modes:

A. Sexual Reproduction

Sexual reproduction involves the formation of seeds through the fusion of male and female gametes. It introduces genetic variability, essential for evolution and breeding.

Processes in Sexual Reproduction:

1. Sporogenesis:
   • Microsporogenesis:
     • Formation of microspores (pollen grains) in the anther.
     • Example: In wheat (Triticum aestivum), microspores form through meiosis in pollen mother cells.
   • Megasporogenesis:
     • Formation of megaspores in the ovule.
     • Example: In rice (Oryza sativa), a single megaspore develops into a functional embryo sac.
2. Gametogenesis:
   • Formation of gametes (male sperm and female egg cells).
   • Example: In maize (Zea mays), male gametes are formed in pollen grains, and female gametes are formed in the ovule.
3. Fertilization:
   • Syngamy: Fusion of sperm and egg nuclei to form a diploid zygote.
   • Triple Fusion: Fusion of a second sperm with the polar nuclei to form a triploid endosperm.
   • Example: In pea (Pisum sativum), fertilization results in seed development.
4. Seed Development:
   • The zygote develops into an embryo, and the ovule matures into a seed.
   • Example: In sunflowers (Helianthus annuus), seeds develop after fertilization.

Advantages of Sexual Reproduction:

• Introduces genetic variability.
• Provides adaptability to changing environments.
• Basis for hybrid breeding programs.

Disadvantages of Sexual Reproduction:

• Requires pollinators or external agents for cross-pollination.
• Longer time for seed formation.

B. Asexual Reproduction

Asexual reproduction involves the multiplication of plants without the fusion of male and female gametes. It results in offspring that are genetically identical to the parent (clones). Asexual reproduction is broadly classified into two types:

1. Vegetative Reproduction
2. Apomixis

1. Vegetative Reproduction Vegetative reproduction refers to the multiplication of plants through vegetative plant parts such as stems, roots, and leaves. It is further divided into natural vegetative reproduction and artificial vegetative reproduction.

a) Natural Vegetative Reproduction This occurs naturally in plants and involves various plant parts such as underground stems, sub-aerial stems, roots, and bulbils.

i) Underground Stems Modified stems that grow below the soil surface and help in reproduction.

• Rhizome: Horizontal underground stem that produces new plants. Examples: Turmeric (Curcuma domestica), Ginger (Zingiber officinale).
• Tuber: Swollen underground stem storing nutrients. Examples: Potato (Solanum tuberosum).
• Corm: Short, thickened underground stem surrounded by protective scales. Examples: Arvi (Colocasia esculenta), Bunda (Colocasia antiquorum).
• Bulb: Underground structure consisting of fleshy leaf bases surrounding a bud. Examples: Garlic (Allium sativum), Onion (Allium cepa).

ii) Sub-Aerial Stems These stems grow horizontally above or just below the soil and give rise to new plants.

• Runner: Produces roots and shoots at nodes. Examples: Strawberry, Grass.
• Sucker: Lateral shoot that arises from the base of the stem. Examples: Banana, Mint (Mentha sp.).
• Stolon: Arching stem that touches the ground to form new plants. Examples: Jasmine, Strawberry.

b). Bulbils

Modified floral structures or buds that develop into new plants upon detachment and falling to the ground. Example: Garlic (Allium sativum).

b) Artificial Vegetative Reproduction This involves human intervention to propagate plants using vegetative parts.

a) Stem Cuttings Plant stems are cut into pieces, each capable of producing roots and shoots. Examples: Sugarcane (Saccharum sp.), Grapes (Vitis vinifera), Roses.

b) Root Cuttings Roots are used to propagate new plants. Examples: Sweet Potato, Citrus, Lemon.

C). Layering A branch of the plant is bent and covered with soil, leading to root formation while still attached to the parent plant. Examples: Jasmine, Guava.

d) Grafting The process of joining parts of two plants (scion and rootstock) to grow as one plant. Examples: Mango, Apple, Rose.