In aquatic environments, external fertilization predominates. Eggs and sperm are released into water where fertilization occurs. This is common in fish, amphibians, most marine invertebrates, and aquatic plants.

Oviparous animals lay eggs with protective shells (calcareous in birds, leathery in reptiles). The amniotic egg with shell evolved as an adaptation to terrestrial life, protecting the embryo from desiccation and physical damage.

The corpus luteum ('yellow body') is named for its yellowish colour due to the accumulation of lipids and lutein (a yellow carotenoid pigment) in the granulosa cells after they transform from the ruptured follicle.

At the onset of labour, progesterone levels fall dramatically (as the corpus luteum regresses and placental progesterone support declines). This removes the inhibitory effect of progesterone on uterine contractions, allowing labour to begin.

The foetal pituitary gland secretes ACTH (adrenocorticotrophic hormone) which stimulates the foetal adrenal cortex to produce cortisol. Cortisol from the foetus helps trigger the onset of labour at term.

The placenta secretes several hormones including human placental lactogen (HPL/chorionic somatomammotropin), HCG (maintains corpus luteum), oestrogen, and progesterone. HPL prepares mammary glands for lactation.

FSH (Follicle Stimulating Hormone) from the anterior pituitary stimulates primary follicles to grow and develop. FSH causes granulosa cells to proliferate, secrete oestrogen, and surround the developing oocyte.

Uterine tubes (Fallopian tubes/oviducts) open at their distal end near the ovary (with fimbriae to catch the egg) and at their proximal end into the uterine cavity. Thus they connect the peritoneal cavity to the uterus.

Rising oestrogen from the dominant Graafian follicle exerts positive feedback on the pituitary, triggering a surge of LH (luteinising hormone). This LH surge causes ovulation around day 14 of the menstrual cycle.

The human gestation period is approximately 280 days (40 weeks) calculated from the first day of the last menstrual period. This equals approximately 9 months, divided into three trimesters.

Gonadotrophins (FSH and LH) from the anterior pituitary regulate both the ovarian cycle (follicle growth, ovulation, corpus luteum formation) and the uterine/menstrual cycle through their effects on oestrogen and progesterone.

Fertilization in humans occurs in the proximal (ampullary) part of the Fallopian tube — the widest part near the ovary. The sperm meets the egg here within 24 hours of ovulation. The zygote then travels to the uterus for implantation.

Parthenocarpy (seedless fruit development without fertilization) is a natural form of asexual reproduction in plants. Grafting is an artificial asexual method. Multiple and binary fission are animal/microbial reproduction methods.

Cloning is the production of genetically identical organisms from a single cell by repeated sub-culturing (mitotic division). Plant tissue culture produces clonal plantlets; animal cloning uses nuclear transfer (Dolly the sheep).

The female reproductive system consists of: ovaries (produce eggs and hormones), oviducts/Fallopian tubes (transport eggs, site of fertilization), uterus (embryo development), cervix, vagina, and external genitalia.

Unisexuality (dioecy — separate male and female individuals) is the advanced mode of animal reproduction. It maximises genetic recombination by preventing self-fertilization. Most complex animals are unisexual.

Vernalisation is the process by which exposure to low temperature promotes subsequent flowering. Plants that require vernalisation include winter cereals (wheat, rye), carrots, and foxglove. The treated plants are called vernalised.

Grafting is an artificial asexual propagation method where a shoot (scion) is joined to a rooted plant (rootstock). It is widely used in horticulture for fruit trees, roses, and vines to combine desirable traits.

Parthenogenesis (development without fertilization), budding (outgrowth that becomes new organism), and vegetative propagation (using roots, stems, leaves) are all forms of asexual reproduction producing clonal offspring.

Isomorphic alternation of generations occurs when the sporophyte and gametophyte generations are morphologically similar (same vegetative structure). Heteromorphic alternation is when they look different (like in ferns).

Test tube babies (IVF — In Vitro Fertilization) is a major reproductive biotechnology achievement. Eggs are fertilized outside the body in a laboratory dish, the resulting embryo is implanted in the uterus, allowing infertile couples to have children.

The human embryo/foetus is enclosed in the amniotic sac (amnion) — a membrane filled with amniotic fluid. Amniotic fluid cushions the foetus against physical shock, allows movement, and maintains constant temperature.

Genital herpes is caused by Herpes Simplex Virus type 2 (HSV-2). It causes painful blisters and ulcers in the genital area. It is a lifelong infection — the virus remains dormant in nerve ganglia and can reactivate.

The after-birth (delivery of the placenta) is the third stage of labour — after the baby is born, the placenta and foetal membranes separate from the uterine wall and are expelled within 30 minutes of birth.

AIDS (caused by HIV) cannot be cured by medication — antiretroviral drugs manage it but don't cure it. Gonorrhoea and syphilis (bacterial) are treated with antibiotics. Genital herpes (viral) can be managed but not cured with antivirals.

Oxytocin from the posterior pituitary stimulates uterine contractions during labour in a positive feedback loop. As the baby's head presses on the cervix, more oxytocin is released, intensifying contractions until birth.

From the beginning of the 3rd month (week 9) of pregnancy, the developing human is called a foetus (fetus). Before this it is called an embryo. The transition marks completion of major organ formation (organogenesis).

The human gestation period is approximately 280 days (40 weeks) from the first day of the last menstrual period, or about 266 days from fertilisation. Normal range is 37-42 weeks.

After ovulation, the ruptured Graafian follicle collapses and its granulosa cells (follicle cells) are transformed into the corpus luteum ('yellow body'). The corpus luteum secretes progesterone and oestrogen to maintain the uterine lining.

In humans, egg production is cyclic (one egg per ~28-day menstrual cycle). Sperm production is continuous — males produce millions of sperm daily from puberty throughout adult life, with no cyclic interruption.

Although many primary follicles begin development each month under FSH stimulation, normally only one Graafian (dominant) follicle completes maturation and releases its oocyte at ovulation. The others degenerate (atresia).

The placenta is the specialised organ formed from both maternal (uterine decidua) and foetal (chorion) tissues. It allows exchange of O₂, CO₂, glucose, amino acids, antibodies, hormones, and waste products between mother and foetus.

The oviduct (Fallopian tube) connects the ovary to the uterus. It is also called the uterine tube. After ovulation, the egg is swept into the Fallopian tube by fimbriae. Fertilization normally occurs in the ampulla of the Fallopian tube.

Primordial germ cells divide by mitosis to produce oogonia (the original egg precursor cells). Oogonia then enter meiosis to become primary oocytes, which are arrested in prophase I from before birth until ovulation.

The uterus narrows inferiorly into the cervix (neck of the uterus), which opens into the vagina. The cervix has an os (opening) that dilates during labour. It also produces mucus that changes consistency during the menstrual cycle.

Normally, only one egg (ovum) is released from one ovary during each ovulatory cycle (ovulation occurs around day 14 of the 28-day cycle). Occasionally two are released, which can result in fraternal twins if both are fertilised.

The female reproductive system includes: ovaries (egg production and hormone secretion), oviducts/Fallopian tubes (egg transport, site of fertilization), uterus (implantation and fetal development), vagina, and external genitalia (vulva).

The male reproductive system consists of: testes (sperm and testosterone production), a duct system (epididymis, vas deferens, ejaculatory ducts, urethra), accessory glands (seminal vesicles, prostate, Cowper's glands), and the penis.

The vas deferens (ductus deferens) is the main muscular duct that transports mature sperm from the epididymis to the ejaculatory duct during ejaculation. Each vas deferens is about 30 cm long. Vasectomy cuts this duct for male sterilisation.

Ovoviviparous animals have internal fertilization and the egg is retained inside the mother, but the embryo is nourished by yolk (not placenta). Examples include some sharks, snakes, and some lizards. The young hatch internally or externally.

Both frogs and fish (most species) have external fertilization — eggs are released into water and sperm are shed over them. Fertilization occurs outside the female's body. This requires a moist aquatic environment.

Fertilization is the specific process of union of male and female gametes (sperm and egg) to form a diploid zygote. It restores the chromosome number and initiates embryonic development.

Parthenogenesis is the development of an egg into an individual without fertilization by sperm. It occurs naturally in bees, wasps, aphids, some lizards, and is induced artificially in frogs by pricking with a needle.

In honey bees, males (drones) develop from unfertilized eggs by arrhenotoky (a form of parthenogenesis). They are haploid (n). Female workers and queens develop from fertilized eggs and are diploid (2n).

Unisexuality (separate sexes, dioecy) is considered the more advanced mode of sexual reproduction in animals, as it prevents self-fertilization and promotes genetic variation. Hermaphroditism (bisexuality) is more primitive.

Clones are organisms produced from a single cell by sub-culturing (mitotic division). They are genetically identical to each other and to the parent. Plant tissue culture produces millions of identical plantlets from a single explant.

Common asexual reproduction methods include tissue culturing (growing plants from cell clusters), cloning (producing genetically identical organisms), and natural methods like budding, fragmentation, and spore formation.

Phytochrome is the photoreceptor pigment that detects red and far-red light, mediating photomorphogenesis and photoperiodism. Cryptochromes are the blue-light receptors. Phytochrome exists in two interconvertible forms (Pr and Pfr).

Apomixis in flowering plants is reproduction without fertilization — a form of plant parthenogenesis. Seeds form from the ovule without fertilization, producing offspring genetically identical to the mother plant. Common in dandelions and mangoes.

Asexual reproduction is considered the more primitive (evolutionarily older) form of reproduction. It is simpler, requires less energy, and was the primary reproduction method of early life forms before sexual reproduction evolved.

In vernalisation, the low temperature stimulus is perceived by the shoot apex (in growing plants) and by the embryo (in seeds undergoing cold stratification). The meristematic cells are the sensors of cold.

The quiescent (inactive) form of phytochrome is Pr (P660) — it absorbs red light (660 nm). When Pr absorbs red light it converts to Pfr (P730, active form) which triggers photomorphogenic responses.

Vernalin — the hypothetical flowering substance induced by cold in vernalised plants — is now believed to be gibberellin. Gibberellins applied to biennial plants can substitute for vernalisation and induce flowering.

Photomorphogenesis is the light-mediated development of plant form and structure. The phytochrome system controls all these developmental responses to light, switching from etiolated (dark-grown) to normal (light-grown) development.

Long-day plants flower when the night length (dark period) is shorter than a critical maximum (i.e., day length exceeds the critical minimum). Examples include wheat, barley, spinach, and clover.

Far-red light (730 nm) converts Pr (phytochrome active in red light) back to Pfr — reversing the effect of red light. Far-red light given after red light restores the dark-grown condition, inhibiting flowering in long-day plants.

Photomorphogenesis is light-controlled development of plant form. It includes: greening (chloroplast development), de-etiolation (inhibition of excessive internode elongation), and unhooking of plumule (apical hook straightening when light detected).

Long-day plants (actually short-night plants) require a dark period shorter than a critical length to flower. Interrupting the long night with even a brief flash of red light allows them to flower, as they perceive it as a short night.

Both photoperiod (day length) and temperature affect flowering (photoperiodism and vernalization), seed production, leaf fall (abscission — triggered by shortening days), and dormancy induction in plants.

Garner and Allard (1920) discovered photoperiodism while studying Maryland Mammoth tobacco (Nicotiana tabacum). They found it only flowered when day length was short, leading to the concept of short-day and long-day plants.

Fruit set refers to the retention of the ovary (and its development into a fruit) following fertilization. Without fertilization, the ovary typically abscises (drops). Auxins from seeds signal the ovary to be retained.

Ethylene (ethene) is the primary hormone associated with fruit ripening. It triggers colour changes, softening, aroma development, and sugar accumulation. Commercially, ethylene gas is used to ripen bananas and tomatoes.

Developing seeds are rich in all three growth hormones — auxins (IAA), gibberellins, and cytokinins. These hormones from seeds stimulate fruit development (fruit set) and suppress leaf/fruit abscission.

Parthenocarpy is fruit development without fertilization and therefore without seed formation. Seedless bananas, seedless watermelons, and navel oranges are parthenocarpic. Auxin and gibberellin application can induce parthenocarpy.

The evolution of the pollen tube parallels independence from water for reproduction — pollen tubes allow sperm to reach the egg without swimming through water, a critical adaptation for terrestrial seed plant reproduction.

Triple fusion occurs in flowering plants (angiosperms) during double fertilization. One sperm (n) fuses with two polar nuclei (2n) forming the triploid (3n) endosperm nucleus. This triple fusion produces the nutritive endosperm.

Double fertilization is unique to angiosperms (flowering plants). One sperm fertilises the egg (→ zygote, 2n) and another fuses with the two polar nuclei (→ endosperm, 3n). This is a defining characteristic of flowering plants.

Asexual reproduction, while efficient, produces genetically identical clones. If the environment changes suddenly (new disease, climate change), lack of genetic variation means all individuals are equally vulnerable, threatening species survival.

Meiosis is called reduction division because it reduces the chromosome number from diploid (2n) to haploid (n). It consists of two divisions: Meiosis I (reduction division) and Meiosis II (equatorial division). Mitosis is the equational division.

All three — fission (binary/multiple), sporulation (asexual spore formation), and budding — are forms of asexual reproduction producing offspring genetically identical (clones) to the parent.

A zygote is the fertilized egg — the diploid cell formed by the fusion of a haploid sperm and a haploid egg. The zygote is the first cell of the new organism and undergoes cleavage to form the embryo.

Parthenogenesis is development of an egg into an individual without fertilization by a sperm. It occurs naturally in honey bees (males/drones from unfertilized eggs), aphids, Komodo dragons, and some sharks.

Frogs (and most amphibians) have external fertilization — eggs are released into water and sperm are shed over them. Rabbits and rats are mammals with internal fertilization. Starfish also have external fertilization.

Elephants are the slowest breeders among large vertebrates — they have the longest gestation period (22 months), reach sexual maturity at 10-12 years, and produce only one calf every 4-5 years, making population recovery very slow.

The testis (plural: testes) is the primary male reproductive organ (gonad). It produces spermatozoa (sperm cells) by spermatogenesis and secretes testosterone and other androgens.

Gibberellins stimulate flowering in long-day plants grown under short-day conditions, promote bolting (rapid stem elongation) in rosette plants, and stimulate fruit development. They also promote pollen germination.

The radicle is the embryonic root — the first structure to emerge during seed germination. It grows downward (positive geotropism) to anchor the seedling and absorb water and minerals from the soil.

During germination, the plumule (embryonic shoot) develops into the shoot system (stem and leaves) of the new plant. The radicle develops into the root system. Together they form the seedling.

FSH (Follicle Stimulating Hormone) from the anterior pituitary stimulates the growth and development of ovarian follicles. FSH causes follicle cells to proliferate and begin oestrogen secretion.

The menstrual cycle (28 days) has four phases: menstrual phase (days 1-5, shedding of endometrium), follicular/proliferative phase (days 6-13, follicle growth, oestrogen rise), ovulatory phase (day 14), and luteal/secretory phase (days 15-28, corpus luteum, progesterone).

The ovary contains germ cells (oogonia that develop into oocytes/eggs) and follicle cells (granulosa cells that surround and nourish the oocyte). The follicle cells also produce oestrogen and progesterone.

The testes have two functions: producing spermatozoa (in seminiferous tubules by spermatogenesis) and secreting testosterone (from Leydig/interstitial cells). Testosterone is the primary male sex hormone.

As Graafian follicles (mature follicles) develop in the ovary, the surrounding follicle cells secrete oestrogen. Rising oestrogen stimulates the LH surge that triggers ovulation.

Sperm are produced in seminiferous tubules → pass to epididymis (maturation and storage) → vas deferens (transport) → ejaculatory duct → urethra (to outside). Seminiferous tubules are the first structure in this pathway.

The male reproductive system consists of testes (in scrotum), a duct system (epididymis, vas deferens, ejaculatory duct, urethra), accessory glands (seminal vesicles, prostate, Cowper's glands), and external genitalia (penis, scrotum).

The placenta is the interface between mother and foetus — it allows selective exchange of oxygen, nutrients (glucose, amino acids), and waste products (CO₂, urea) while preventing mixing of maternal and fetal blood.

In human females, the ovaries (gonads) are paired organs located in the lower abdominal cavity (pelvis), one on each side of the uterus. They are held in place by ligaments.

The human gestation period is approximately 280 days (40 weeks or 9 months plus 7 days) from the first day of the last menstrual period. This is conventionally divided into three trimesters of approximately 13 weeks each.

Oocytes (developing egg cells) are located in sac-like follicles in the ovary. Each follicle contains one oocyte surrounded by follicle cells that nourish and protect it during maturation.

Genitalia refers to the reproductive organs — both internal (gonads, ducts, accessory glands) and external (vulva in females, penis and scrotum in males). The term encompasses all reproductive structures in both sexes.

Monotremes (egg-laying mammals like platypus and echidnas) lay small, leathery eggs with yolk for embryonic nutrition — similar to reptile eggs. After hatching, young are nursed with milk secreted through skin pores.

In birds, the chorion fuses with the allantois to form the chorioallantoic membrane — a highly vascularised structure just beneath the shell that serves as the main organ of gas exchange and calcium absorption from the shell.

The amnion is the innermost extra-embryonic membrane that surrounds and encloses the amniotic cavity filled with amniotic fluid. It protects the embryo from physical shock and desiccation.

The uterus (womb) is the muscular organ that provides a safe, warm, moist environment for the developing embryo and fetus in viviparous animals. The placenta connects the fetus to the uterine wall for nutrient exchange.

Viviparous animals give birth to live young that develop inside the mother's uterus, nourished by placenta. All placental mammals are viviparous. The embryo receives nutrients and oxygen directly from the mother's blood.

Oviparous animals lay eggs in which the embryo develops outside the mother's body, nourished by yolk. Examples include birds, reptiles, fish, amphibians, and monotreme mammals (platypus, echidna).

Fission is the splitting of a parent organism into two or more daughter organisms. Binary fission (in bacteria, Amoeba) produces two equal daughters; multiple fission (in Plasmodium) produces many daughters simultaneously.

Desert (terrestrial) animals that lay eggs have evolved amniotic eggs with shells and protective membranes (amnion, chorion, allantois) that prevent desiccation. Aquatic animals can lay unprotected eggs in water.

Internal fertilization requires a copulatory organ (penis) through which male deposits sperm directly into the female's reproductive tract. This allows fertilization without water and evolved as an adaptation to terrestrial life.

Reproduction is of two fundamental types: asexual (without gamete fusion — produces genetically identical offspring) and sexual (involves gamete fusion — produces genetically varied offspring). Both are essential for species survival.

Short day plants (actually long night plants) flower when the day length is shorter than a critical minimum. Examples include chrysanthemum, cocklebur, tobacco (Maryland Mammoth), and poinsettia.

Vernalisation is the process by which low temperature (applied to seeds or growing plants) shortens the vegetative phase and induces earlier flowering. It was discovered in winter cereals that need cold before they flower.

Gibberellins prevent genetic/physiological dwarfism by promoting cell elongation, especially in stems. They stimulate internode elongation, which is deficient in dwarf varieties of plants due to low gibberellin production.

Leaves perceive the photoperiodic stimulus (day length). The flowering hormone (florigen, now identified as Flowering Locus T protein) is produced in leaves in response to appropriate day length and transported to the shoot apex.

Auxin (at optimal concentrations) promotes rapid cell division and elongation, especially in the apical meristem. Auxin activates cell wall loosening enzymes (expansins) allowing water uptake and cell expansion.

In higher plants, growth is localised at specific regions called meristems (growth points) — apical meristems (tips of roots and shoots) and lateral meristems (cambium). Growth is not distributed throughout the plant.

Gibberellins were first extracted from the fungus Gibberella fujikuroi (which causes 'bakanae' or foolish seedling disease in rice) by Yabuta and Hayashi in 1938. They cause abnormal elongation of rice seedlings.

Plant growth inhibitors include abscisic acid (ABA — promotes dormancy, closes stomata, inhibits growth) and ethylene (promotes fruit ripening, leaf abscission, and inhibits stem elongation).

Vascular cambium and phellogen (cork cambium) are the two lateral meristems responsible for secondary growth (increase in girth) in dicot plants. They produce secondary xylem and phloem, and cork respectively.

Cell enlargement (cell expansion) is important in tissue formation and growth. After cell division, cells enlarge by absorbing water into vacuoles, increasing in size up to 150 times, which contributes most to plant growth in volume.

Cutting and grafting are the two main artificial vegetative propagation methods. Cuttings involve rooting a detached stem, leaf, or root. Grafting joins a shoot (scion) to a rooted plant (rootstock) for growth.

Aggregated fruits (multiple fruits or composite fruits) develop from an inflorescence (cluster of flowers) — examples include pineapple (from a spike inflorescence), fig, mulberry, and breadfruit.

A pome (e.g., apple, pear) is a false/accessory fruit where the fleshy part develops from the receptacle (thalamus), not the ovary. The true fruit is the core containing seeds enclosed in papery endocarp.

True fruits develop exclusively from the ovary wall (pericarp) after fertilization. False fruits (pseudocarps) incorporate other floral parts — like the apple (fleshy receptacle) or strawberry (receptacle).

Fertilization is the fusion of two dissimilar gametes (anisogamy) — typically a small, motile sperm and a large, non-motile egg — to form a diploid zygote. This restores the full chromosome number.

The second type of asexual reproductive bodies (spores) are produced by the sporophyte (diploid) generation through meiosis. These haploid spores germinate to form the gametophyte (haploid) generation.

Sexual reproduction has two fundamental patterns: isogamy (fusion of morphologically similar gametes) and anisogamy (fusion of morphologically different gametes, including oogamy where egg and sperm are involved).

Zoospores are the first/simplest type of asexual reproductive units — they are motile (flagellated) spores produced by algae (like Chlamydomonas) and some fungi. They swim through water to new locations to germinate.

Cutting (taking stem/leaf pieces and rooting them) and grafting (joining a stem/bud to a rootstock) are artificial methods of vegetative propagation used by horticulturists to rapidly produce genetically identical plants.

There are three ways of reproduction: asexual reproduction (without gametes), sexual reproduction (involving gametes), and vegetative reproduction (using vegetative parts of a plant). Some classify these as just two: sexual and asexual.

Inflorescence is the arrangement of flowers on the floral axis or stem. Phyllotaxy is leaf arrangement. Aestivation is the arrangement of floral parts in a bud. Vernation is the arrangement of leaves in a bud.

Inflorescences (flower arrangements) are of two main types: cymose (cyme — determinate, where the terminal bud becomes a flower first) and racemose (raceme — indeterminate, where the main axis continues to grow).

The endosperm is triploid (3n) — formed by the fusion of one sperm (n) with two polar nuclei (2n) during triple fusion in double fertilization of angiosperms. It provides nutrients for the developing embryo.

Cleistogamy is self-pollination that occurs within a permanently closed (cleistogamous) flower that never opens. It ensures seed production when cross-pollination is not possible. Examples include violet and peanut.

The endosperm is formed from the definitive nucleus (result of triple fusion — two polar nuclei + one sperm nucleus) in angiosperms. It is triploid (3n) and provides nutrition for the developing embryo.