Morphology and Sexual Reproduction in Angiosperms

Flower Morphology and Structure

• Flowers are highly adapted to ensure formation of fruits and seeds, the end products of sexual reproduction in angiosperms
• Humans have had an intimate relationship with flowers since ancient times, using them for aesthetic, ornamental, social, religious, and cultural purposes

Parts of a Flower

• A typical flower consists of stigma, style, ovary, and thalamus
• Ovary contains ovules, which are attached to the placenta by a stalk called funicle
• Ovules have one or two protective envelopes called integuments, which encircle the nucellus except at the tip where a small opening called the micropyle is organized

Structure of an Ovule

• Each ovule has a small stalk called funicle, which fuses with the body of the ovule at the hilum
• Integuments encircle the nucellus, which contains reserve food materials
• Located in the nucellus is the embryo sac or female gametophyte

Pollination and Fertilisation

• Emasculated flowers are covered with a bag to prevent contamination with unwanted pollen
• Pollen grains are collected from the male parent and dusted on the stigma, and the flowers are rebagged
• Pollen-pistil interaction involves events from landing of pollen grains on the stigma to the pollen tube entering the embryo sac

Double Fertilisation

• Double fertilisation occurs in flowering plants, involving two fusion events: syngamy and triple fusion
• One male gamete fuses with the egg cell to form a diploid zygote, and the other male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus

Seed and Fruit Formation

• As ovules mature into seeds, the ovary develops into a fruit
• Seed coats are formed from hardened integuments, with the micropyle remaining as a small pore
• Fruits may be fleshy or dry and have evolved mechanisms for dispersal of seeds
• In some species, the thalamus also contributes to fruit formation, forming false fruits