32.2 Pollination and Fertilization

Pollination and Fertilization in Plants

Pollination and fertilization are the processes that allow flowering plants to reproduce sexually. Pollination gets pollen from the male structure to the female structure, and fertilization is the actual fusion of gametes that follows. Together, they lead to seed and fruit formation.

Reproductive Structures and Gamete Formation

Before diving into pollination, you need to know the structures involved.

The stamen is the male reproductive structure, made up of two parts:

• The anther, which produces pollen grains
• The filament, which supports the anther

The pistil (also called the carpel) is the female reproductive structure, with three parts:

• The stigma, a sticky surface at the top that catches pollen
• The style, a tube connecting the stigma to the ovary
• The ovary, which contains one or more ovules

Gametes form through specific processes within these structures:

• Microsporogenesis occurs in the anther, producing microspores that develop into pollen grains. Each mature pollen grain contains the male gametophyte, which will eventually produce two sperm cells.
• Megasporogenesis occurs in the ovule, producing a megaspore that develops into the embryo sac (the female gametophyte). The embryo sac contains the egg cell and two polar nuclei, both of which play roles in fertilization.

Self-Pollination vs. Cross-Pollination

Self-pollination is the transfer of pollen from the anther to the stigma of the same flower, or to another flower on the same plant. Peas, wheat, and tomatoes are common self-pollinators. Because offspring inherit genes from only one parent, self-pollination produces genetically uniform offspring. That's an advantage in stable environments where the parent is already well-adapted, but it limits the population's ability to respond to new challenges.

Cross-pollination is the transfer of pollen from the anther of one plant to the stigma of a different plant of the same species. Apple trees, sunflowers, and grasses rely on cross-pollination. This requires an external agent to move the pollen:

• Wind carries lightweight pollen (grasses, many trees)
• Water transports pollen in some aquatic plants
• Insects like bees visit flowers for nectar and pick up pollen along the way
• Birds such as hummingbirds transfer pollen while feeding

Cross-pollination produces genetically diverse offspring because genes come from two different parents. This diversity is a major advantage in changing environments, giving populations more variation for natural selection to act on.

From Pollination to Seed Development

Here's the full sequence from pollen landing on the stigma to a mature seed inside a fruit:

1. Pollination occurs when pollen grains land on a compatible stigma.

2. Pollen germination and tube growth begin as the pollen grain absorbs moisture and nutrients from the stigma. A pollen tube emerges and grows down through the style toward the ovary. Chemical signals released by the ovule guide the tube to its target.

3. Double fertilization takes place once the pollen tube reaches the ovule and releases two sperm cells into the embryo sac:

   • The first sperm fuses with the egg cell, forming a diploid zygote ($2n$)
   • The second sperm fuses with the two polar nuclei, forming the triploid endosperm ($3n$)

4. Seed development follows fertilization:

   • The zygote divides and develops into an embryo, which will become the new plant
   • The endosperm accumulates nutrients that feed the developing embryo
   • The outer layers of the ovule harden into a seed coat, protecting the embryo

5. Fruit development occurs as the ovary wall matures into a fruit that encloses the seed(s). Fruits like apples, oranges, and cucumbers aid in seed dispersal by attracting animals or using other mechanisms.

Double Fertilization in Flowering Plants

Double fertilization is unique to angiosperms (flowering plants) and is one of the features that distinguishes them from gymnosperms. The key idea is that both sperm cells delivered by the pollen tube are used, each fusing with a different cell in the embryo sac.

• Sperm + egg → zygote → embryo
• Sperm + two polar nuclei → endosperm (nutrient tissue)

Why does this matter? Double fertilization links embryo development and food reserve production to the same event. The endosperm only forms when fertilization actually occurs, so the plant doesn't waste energy building a food reserve for an unfertilized ovule. This efficiency is considered a key adaptation in the evolutionary success of angiosperms.

The resulting seed is a complete survival package: an embryo, a food supply (endosperm), and a protective seed coat. This combination allows seeds to disperse to new locations and survive until conditions are right for germination.