Flowering Plants: Sexual Reproduction

• Sexual reproduction in flowering plants involves the fusion of male and female gametes to produce offspring with genetic variation.

Plant Reproductive Structures

• The flower is the primary reproductive structure in flowering plants (angiosperms).
• Flowers contain both male and female reproductive organs, or only one of the two.
• The male reproductive organ is the stamen, consisting of the anther (where pollen is produced) and the filament (which supports the anther).
• The female reproductive organ is the pistil, composed of the stigma (which receives pollen), the style (a tube connecting the stigma to the ovary), and the ovary (which contains the ovules).
• Some flowers have petals and sepals, which are sterile floral parts.
• Petals attract pollinators, while sepals protect the developing flower.

Pollination mechanisms

• Pollination is the transfer of pollen from the anther to the stigma of a flower.
• Pollination can be achieved through various mechanisms, including wind, water, insects, birds, and other animals.
• Wind-pollinated flowers typically produce large amounts of lightweight pollen.
• They often have reduced or absent petals and sepals.
• Insect-pollinated flowers often have bright petals, strong fragrances, and nectar to attract pollinators.
• Bird-pollinated flowers are often red or orange, tubular in shape, and produce large amounts of nectar.
• Self-pollination occurs when pollen is transferred from the anther to the stigma of the same flower or another flower on the same plant.
• Cross-pollination occurs when pollen is transferred from the anther of one flower to the stigma of a flower on a different plant of the same species.
• Cross-pollination promotes genetic diversity.

Fertilization Processes

• Fertilization in flowering plants is a double fertilization process.
• After pollination, a pollen grain lands on the stigma and germinates, forming a pollen tube.
• The pollen tube grows down the style and into the ovary, eventually entering an ovule through the micropyle.
• The pollen grain contains two sperm cells.
• One sperm cell fuses with the egg cell to form a diploid zygote, which will develop into the embryo.
• The second sperm cell fuses with the central cell, which contains two polar nuclei, to form a triploid endosperm.
• The endosperm serves as a food reserve for the developing embryo.

Seed Development

• After fertilization, the ovule develops into a seed.
• The zygote develops into an embryo, which consists of the embryonic root (radicle), the embryonic shoot (plumule), and one or two seed leaves (cotyledons).
• The endosperm provides nutrients to the developing embryo.
• The integuments of the ovule develop into the seed coat (testa), which protects the embryo.
• As the seed matures, it dehydrates and becomes dormant.
• The ovary develops into a fruit, which encloses and protects the seeds.
• Fruits can be fleshy (e.g., berries, drupes) or dry (e.g., legumes, nuts).
• Fruits aid in seed dispersal through various mechanisms, such as wind, water, and animals.

Genetic Variation In Offspring

• Sexual reproduction in flowering plants results in offspring with genetic variation due to several factors.
• Independent assortment of chromosomes during meiosis shuffles the genetic material.
• Recombination (crossing over) during meiosis exchanges genetic material between homologous chromosomes.
• Random fusion of gametes during fertilization combines genetic material from two different parents.
• These processes lead to new combinations of genes in the offspring, resulting in genetic variation.
• Genetic variation is important for adaptation to changing environments and for the evolution of new traits.