Fertilization

Fertilization in plants

Definition: Fertilization is the union of the nuclei of male and female reproductive cells (gametes), resulting in offspring that are genetically unique.

Importance:

1. Produces genetically diverse offspring, aiding adaptation to new environments.
2. Often leads to the formation of seeds, which facilitate species dispersal and reduce competition.

Events leading to fertilization in plants

Plants can be divided into two groups based on the type of spores they produce:

Homosporous plants

1. Produce one type of spore, identical in size and shape.
2. Each spore can develop into a gametophyte bearing both male (antheridia) and female (archegonia) reproductive structures.
3. Examples: Bryophytes and most ferns.

Spore production:

1. Spores are produced in sporangia via meiosis.
2. In bryophytes, sporangia are located inside a capsule.
3. In ferns, sporangia are found in sori on the underside of fronds.
4. Spores are lightweight and dispersed by wind to germinate under favorable conditions.
5. The germinated spore forms a gametophyte with antheridium (produces sperm) and archegonium (produces eggs).

Heterosporous plants

Produce two types of spores:

1. Microspores (male).
2. Megaspores (female).

These develop into separate male and female gametophytes.

Double fertilization in higher plants (Angiosperms)

Angiosperms undergo a unique process called double fertilization:

Pollination

Definition: Transfer of pollen grains from the anthers to the stigma.

Types:

1. Self-pollination: Pollen from the same flower or plant fertilizes the ovule.
2. Cross-pollination: Pollen from one plant fertilizes the ovule of another plant of the same species.

Pollination is facilitated by agents like wind, insects, birds, and mammals (collectively called pollinators).

Fertilization process

1. Pollen germination:
   • Pollen grains land on a compatible stigma and absorb a sugary solution secreted by the stylar tissues.
   • The intine of the pollen grain grows through an aperture in the exine, forming a pollen tube.
2. Pollen tube growth:
   • The pollen tube nucleus directs tube growth towards the ovule, controlled by auxins secreted by stylar tissues.
   • The growth is guided by chemicals secreted by synergids in the embryo sac, a process known as chemotactic growth.
3. Male gamete formation: The generative nucleus in the pollen grain divides by mitosis, producing two haploid male gametes.
4. Entry into the ovule: The pollen tube enters the ovule through the micropyle and releases the two male gametes into the embryo sac.

Double fertilization

Angiosperms exhibit a unique process called double fertilization, involving two distinct fertilization events:

1. Entry into the ovule:
   • The pollen tube enters the ovule through the micropyle.
   • The tip of the tube bursts, releasing the two haploid male gametes into the embryo sac.
2. First fertilization event:
   • One male gamete fuses with the egg cell (haploid) to form a zygote (diploid), which develops into the embryo.
3. Second fertilization event:
   • The second male gamete fuses with the two polar nuclei (already fused into a diploid structure) to form a triploid (3n) endosperm.
   • The endosperm serves as a nutritive tissue for the developing embryo.

Endosperm formation and function

The endosperm stores food for the developing seed, mainly in the form of:

• Carbohydrates: Starch and sugars.
• Proteins and lipids: Found in seeds like sunflower and nuts.

Significance of double fertilization

1. Ensures the development of a nutritive tissue (endosperm) only when fertilization has occurred, preventing waste of resources.
2. Enhances seed viability by providing nourishment during early growth stages.
3. Contributes to the evolutionary success of angiosperms by enabling efficient reproduction.