Sexual Reproduction in Plants

• Sexual reproduction involves specialized sex cells called gametes.
• Gametes are usually haploid.
• During sexual reproduction, the nuclei of two gametes fuse together in fertilization.
• The product of fertilization is a diploid cell named a zygote.
• A zygote divides repeatedly by mitosis to produce an embryo, which eventually develops into a mature individual.
• Most species have two types of gametes, one is relatively small and moves named the male gamete, the other is relatively large and does not move named the female gamete.
• The production of gametes usually involves meiosis which introduces genetic variation.
• The reproductive organ of a flowering plant is the flower.
• The male gametes and female gametes are made in the flower and fertilization takes place there.
• The male gametes consist of haploid nuclei inside pollen grains produced in the anthers of the flower.
• Pollen grains contain the male gametes.
• The female gametes are haploid nuclei inside an embryo sac located inside an ovule.
• Male gametes of flowering plants aren't motile and depend on outside agencies to transport their male gametes inside pollen grains from one flower to another.
• Pollination results in the deposition of pollen grains onto a stigma.
• A pollen grain germinates and produces a tube that grows towards an ovule and penetrates an embryo sac.
• The male gametes inside the pollen grain pass down the tube and fuse with the female gametes inside the embryo sac, producing a zygote.
• A zygote divides by mitosis to produce an embryo plant.
• An ovule becomes a seed, containing the embryo.
• The gynoecium consists of one or more ovaries, each containing one or more ovules, inside which the female gametes develop inside an embryo sac.
• Atop each ovary is a style which supports a stigma.
• The stigma has the function of capturing pollen grains.
• The haploid nucleus inside each pollen grain divides by mitosis, forming two haploid cells separated by a thin cell wall.
• One of the haploid nuclei is the generative nucleus, and the other is the tube nucleus.
• After the pollen has landed on a stigma, the generative nucleus divides by mitosis to produce two male gamete nuclei.
• When the pollen grains are fully formed, the anthers split open in dehiscence.
• Haploid tube nucleus and haploid generative nucleus are part of the pollen structure.
• Plasma (cell surface) membrane, pit, intine, and exine, which is very tough and waterproof are also parts of the pollen structure.

Flower Structure

• The different parts of a flower are arranged in concentric rings, called whorls, attached to the top of the flower stalk.
• The part to which they are attached is the receptacle.
• These parts are arranged in order.
• The outer whorl is the calyx and is made up of the sepals.
• In many flowers, sepals are dull in colour and protect the other flower parts when it is in bud.
• The next whorl is the corolla, made up of petals.
• Insect-pollinated or bird-pollinated flowers have brightly colored and scented petals to attract insects / birds
• Petals advertise the presence of the flower and the color/scent may determine the kind of insect or other animal attracted.
• Nectaries secrete a fructose-rich fluid called nectar to attract insects and birds for pollination.

Male and Female parts of the flower

• The next whorl from the petals is made up of the male parts of the flower called the androecium.
• The androecium contains several stamens.
• Each stamen has a stalk called a filament, which supports an anther.
• Inside the anthers, the male gametes are formed inside pollen grains.
• In the center of the flower are the female parts, the gynoecium.
• Male gametes are formed inside pollen grains.
• Pollen grains are formed inside the anthers.
• Each anther contains four compartments called pollen sacs.
• The pollen sac's wall contains several layers of cells.
• One of the outer layers is made up of cells with thickened walls named the fibrous layer.
• The fibrous layer helps to liberate the pollen grains when they are ripe.
• The innermost layer is termed the tapetum that help to provide nutrients to the developing pollen grains.
• In the center of each pollen sac, diploid pollen mother cells divide by meiosis, each producing four haploid cells.
• Some species of haploid cells stay together in a group of four called a tetrad, but in others they separate.
• Each of the haploid cells develops a tough, protective wall around itself, becoming a pollen grain.
• The wall is made up of two layers, an outer, very tough, waterproof exine, containing sporopollenin, and an inner intine.
• There's a thin area in the wall called a pit, where the exine is absent.
• The form and structure of the exine varies from species to species.
• Plants can be identified just by looking at its pollen grains.
• Pollen grains often have spikes or knobs to help stick to bodies of insects or birds.