Sexual Reproduction in Flowering Plants

Imagine a flower is like a small house with four rooms. Outside room sepals green like gate that protects bud. Second room petals colourful like paint to invite guests insects. Third room androecium is male part made of stamens, each stamen has a thin stick filament holding a double bag anther on top where pollen grains live, like a bag of yellow powder. Fourth room gynoecium female part made of carpels.

Anther bag is not simple. If you cut it, it has four small chambers called microsporangia. So two lobed anther has four chambers called dithecous tetrasporangiate. Wall of chamber has four layers from outside to inside: epidermis outside skin, endothecium middle with spring like thickening that helps bag burst when dry, middle layer dies early, and tapetum inside which is like a kitchen, it cooks food for young pollen and makes outer hard wall of pollen sporopollenin. Tapetum cells have many nuclei.

Inside chamber many cells divide and become microspore mother cells 2n, each mother cell does meiosis and makes four pollens in a group called tetrad. So one mother gives four pollens haploid n. This making of pollen is called microsporogenesis. Think mother makes four babies by dividing.

Pollen grain is like a tiny suitcase with two people inside. Outer cover exine very hard made of sporopollenin, toughest material, can survive thousands of years in fossil. It has small pores germ pores from where tube comes out. Inner cover intine soft made of cellulose. Inside there is vegetative cell big which will make tube, and generative cell small which will divide into two male sperms. Some pollens shed in two celled stage vegetative + generative, some in three celled vegetative + two sperms.

Now female side. Carpel has three parts: top stigma sticky like glue to catch pollen, middle style like tube, bottom ovary which has ovules inside like eggs in a nest. Ovule attached to ovary by stalk funicle at point hilum. Main body nucellus food tissue covered by one or two coats integuments leaving small hole micropyle at top for pollen tube entry, bottom is chalaza. Most ovules are anatropous inverted so micropyle near funicle, like head bent down.

Inside nucellus one cell becomes megaspore mother cell MMC 2n, does meiosis and makes four megaspores, three die one survives functional megaspore. This is megasporogenesis, like one mother making four daughters but three sacrificed one lives. Functional megaspore divides three times by mitosis without walls 1→2→4→8 nuclei, then walls form. It becomes embryo sac with 7 cells 8 nuclei: at top near micropyle egg apparatus one egg + two helper synergids which have finger like filiform apparatus to guide tube, at bottom three antipodals, in centre big central cell with two polar nuclei which fuse to secondary nucleus 2n. This embryo sac is female gametophyte, very small but ready to become baby plant.

Pollination is simply delivery of pollen boy to stigma girl. If boy from same flower goes to same flower girl it is autogamy self love, needs flower to be bisexual and anther stigma mature together and close. Some flowers never open called cleistogamous like Viola Oxalis, they always do self pollination because no chance for outside.

If pollen from one flower goes to another flower on same plant, it is geitonogamy, genetically still self because same plant but needs insect or wind to carry. If pollen goes from one plant to another plant of same type, it is xenogamy cross love, brings new characters variation.

How does pollen travel? By wind anemophily: pollen light dry non sticky, anther hangs out, stigma big feathery like broom to catch wind, common in grasses maize wheat. By water hydrophily very rare: Vallisneria female flower comes to water surface male flower floats releases pollen on water, Zostera sea grass pollen thread like underwater. By insects entomophily most beautiful: flowers large colourful fragrant sweet nectar, pollen sticky spiny, stigma sticky, bees butterflies help like sunflower rose jasmine.

Plants avoid self pollination by many tricks: pollen released before stigma ready or stigma before pollen, anther and stigma at different heights herkogamy, chemical self incompatibility where own pollen cannot grow, and making male female flowers separate unisexual.

When compatible pollen lands on stigma, stigma decides if it is right type. If wrong, it blocks. If right, pollen germinates and makes a long tube called pollen tube, like a tunnel from stigma through style to ovule. Vegetative cell makes tube, generative cell divides into two male sperms traveling inside tube.

Tube usually enters ovule through micropyle hole called porogamy most common, sometimes through side mesogamy or bottom chalaza chalazogamy. It enters one synergid and bursts, synergid dies, two sperms released.

Now double fertilization happens, special only in flowering plants, two marriages at same time. One sperm marries egg → forms zygote 2n which will become baby plant, this is syngamy. Second sperm marries two polar nuclei in central cell → forms primary endosperm nucleus PEN 3n triple fusion which will become food for baby. So two fertilizations same time, hence double fertilization. After this, synergids and antipodals degenerate, their job over.

After fertilization, ovary becomes fruit, ovules become seeds. Food tissue endosperm forms first from PEN 3n. Three types: nuclear type nuclei divide without walls free nuclei float like coconut water is free nuclear endosperm later walls form, cellular type wall after each division, helobial intermediate. Coconut water you drink is nuclear liquid endosperm, white kernel is cellular.

Embryo forms from zygote. Dicot embryo has two seed leaves cotyledons plus axis with epicotyl above cotyledons which makes shoot plumule and hypocotyl below which makes root radicle. Monocot has one cotyledon called scutellum that absorbs food from endosperm, plus coleoptile cap over plumule and coleorhiza cap over radicle protective helmets.

If endosperm fully eaten by embryo seed has no endosperm called non-endospermic like pea bean, if remains called endospermic like maize castor coconut. Seed has seed coat from integuments protects embryo. Fruit is ovary wall pericarp with seeds inside. True fruit only from ovary like mango, false fruit ovary plus other parts like apple from thalamus fleshy. Fruit without fertilization seedless called parthenocarpic like banana made by hormone.

Some plants have clever tricks: apomixis making seed without fertilization, so seed is clone of mother, keeps hybrid characters same, useful in agriculture. Polyembryony many embryos in one seed like Citrus mango onion, often nucellus cells form extra embryos called nucellar polyembryony. So one seed gives many plants.