Reproduction: Plant, Human, and Sexually Transmitted Diseases (14.1–14.4)

14.1 Reproduction

  • Reproduction: life process that gives rise to a new generation; essential for continuity of life.
  • Types: asexual reproduction and sexual reproduction.
  • Asexual reproduction: offspring arise from spores or vegetative parts of a matured organism; offspring are genetically similar to the parent (clones); no production of gametes.
  • Sexual reproduction: fusion of two gametes produced in sexual structures of male and female organisms; involves meiosis and genetic recombination; offspring have mixed characteristics.
  • In plants, sexual structure is the flower; in animals, male and female reproductive systems exist.
  • Key contrasts (summary):
    • Meiosis: Asexual reproduction — does not occur; Sexual reproduction — occurs.
    • New species: Asexual — usually not produced; Sexual — can lead to new species with better environmental adaptation.
    • Offspring numbers: Asexual — large numbers in short time; Sexual — slower increase.
    • Occurrence: Asexual — primitive plants/animals; Sexual — plants and evolutionarily advanced animals.
  • Essentials for reproduction to occur: formation of specialized reproductive structures and gametes; fusion of gametes in sexual reproduction leads to variation.

14.2 Reproduction of plants

  • Plant propagation occurs mainly by two methods: asexual (vegetative propagation) and sexual reproduction.
  • Asexual reproduction (vegetative reproduction): generation of new plants from vegetative parts; daughter plants identical to the mother plant.
  • Vegetative propagation is of two types:
    • Natural vegetative propagation
    • Artificial vegetative propagation
  • Natural vegetative propagation uses various vegetative parts; examples:
    • Roots (e.g., Curry leaves, Bread fruit, Guava, Slime wood)
    • Leaves (e.g., Begonia, Akkapana/Sathaikkaraichchan, Queen of the night)
    • Suckers (e.g., Paddy, Banana, Pineapple, Chrysanthemum, Hulankeeriya, Kaladuru)
    • Runners (e.g., Gotukola/Vallarai, Sweet potatoes, Maharaavana raevula/Ravanan meesai)
    • Bulbils (special reproductive structure from a vegetative or flower bud; e.g., Pineapple, Jute, Hondala)
    • Underground stems (rhizome, corm, bulb, stem tuber) for storage and dormancy
  • Underground stems: four types
    • Rhizome (e.g., Ginger, Turmeric, Cannas, Araththa)
    • Corm (e.g., Taro/yam variants such as Habarala/ Nersshembu, Cocoyam)
    • Bulb (e.g., Red onion, Big onion, Leeks)
    • Stem tuber (e.g., Potato, Coleus, Innala)
  • Artificial vegetative propagation: human-made propagation by vegetative means (not seeds), via:
    • Rooting of stem cuttings
    • Layering
    • Grafting
    • Tissue culture
  • Rooting of stem cuttings: propagate by planting cuttings from a healthy mother plant; best with twigs without tender leaves; examples: Rose, Shoeflower, Ixora, Bougainvillea, Croton.
  • Layering: rooting while still attached to mother plant; two types:
    • Ground layering: twig bent and buried in soil to root; examples: Jasmin, Lemon
    • Aerial layering: ring-barked twig with a compost-coir mix, wrapped to root; examples: Pomegranate, Lemon
  • Grafting (twigs or buds): connecting a twig/bud (scion) to a plant of related species (stock) to fuse cambium; stock should have a strong root system, uniform growth, environmental resilience; scion should be a variety with desirable traits and pest/disease resistance.
    • Bud grafting: insert a live bud into a cut on the stock; common bud shapes: T-bud, H-bud, V-bud; steps include inserting bud, wrapping with polythene, and removing wrap to allow bud growth.
    • Twig grafting: use a twig as scion; align cambium; wrap to secure; remove wrap when growth begins.
    • Advantages: offspring with scion traits, disease-resistant root systems, propagation of seedless plants; Disadvantages: shorter lifespan, not compatible with all species, may reduce wood value.
  • Tissue culture (micropropagation): producing new offsprings identical to the mother plant (clones) from vegetative tissue in a culture medium; steps: introduce tissue, form callus, develop plantlets, acclimate to field conditions; advantages: large numbers, rapid propagation, disease-free plant material, potential to combine favorable genes in hybrids.
  • Major advantages of vegetative propagation: propagation of non-seed-bearing plants, identical offspring, early fruiting, disease/pest resistance, environmental resilience.
  • Major disadvantages: reduced genetic variation; new varieties may not evolve as quickly.
  • Flower structure (sexual part of plants): calyx, corolla, androecium (stamen), gynoecium (pistil).
  • Flower parts (function):
    • Calyx: made of sepals; outer protection in bud stage.
    • Corolla: petals; attracts pollinators.
    • Androecium: male reproductive structure; consists of filament and anther; pollen sacs produce pollen (male gametes).
    • Gynoecium: female reproductive structure; stigma, style, ovary; ovules contain female gametes.
  • Bisexual (perfect) flowers: contain both male and female parts (e.g., Shoeflower, Passion fruit, Chilli, Kathurumurunga/Agathi).
  • Unisexual flowers: staminate (male only) or pistilate (female only); examples: sterile male flowers on pumpkins; fruit-bearing female flowers on pumpkins or corn.
  • Monoecious plants: bear both staminate and pistilate flowers on the same plant (e.g., Pumpkin, Corn, Coconut, Bitter gourd).
  • Dioecious plants: staminate and pistilate flowers on separate plants (e.g., Papaw, Vallisneria).
  • Pollination: deposition of matured pollen on the stigma of a flower of the same species; two main types:
    • Self-pollination
    • Cross-pollination
  • Adaptations to promote cross-pollination and avoid self-pollination include:
    • Unisexual flowers (separate male and female flowers on the same plant or different plants)
    • Self-sterility (pollen not capable of fertilizing the same flower)
    • Herkogamy (stamens and stigma positioned apart)
    • Dichogamy (stamen and pistil maturity at different times: proterandry or protogyny)
  • Agents of pollination:
    • Animals (zoophilous) – most insects; traits: fragrance, large, colorful, nectar, sticky pollen/stigma, advantageous flower-animal contact.
    • Wind (anemophilous) – flowers usually separate (staminate and pistilate), large pollen production, very small/light pollen, branched stigma, inflorescences.
    • Water (hydrophilous) – seasonal, often separate male/female flowers; pollen floats to pistillate flowers (e.g., Vallisneria).
  • Artificial pollination: manually transferring pollen to stigma using fingers or a brush.
  • Fertilization: pollen germinates on stigma, pollen tube grows through the style to the ovule, male gamete fuses with the ovum to form a zygote.
  • Fruits and seeds: after fertilization, ovary develops into fruit; ovule forms seed; seed coat forms testa; sometimes sepals become fleshy and attached to the pericarp (e.g., guava, brinjal, mangosteen, rose apple); parthenocarpy: fruit development without fertilization (seedless fruits via growth regulators).
  • Dispersal of fruits and seeds: four main methods to spread offspring and reduce competition/ensure colonization: by animals, by water, by wind, and by explosive mechanisms.
  • Dispersal adaptations by animals: succulent edible parts, attractive colors, hooks/hairs, shapes to fool animals.
  • Dispersal adaptations by wind: threads to float, wing-like structures, apex fruits, very light seeds, large numbers.
  • Dispersal adaptations by water: porous/fibrous pericarps, floatable or air-filled structures (e.g., coconuts, lotuses).
  • Explosive dispersal: some fruits burst to fling seeds far away (e.g., rubber, Ladies fingers).
  • Germination of seeds: activation of embryo to form a seedling; essential factors: viability, oxygen, water/moisture, and suitable temperature.
  • Dormancy: seeds that fail to germinate despite favorable conditions; causes: immature embryo, impermeable seed coat; methods to break dormancy include storage for a period, scarification, removing seed coat, hot water treatment, gentle cracking.
  • Seed germination types:
    • Hypogeal germination: plumule stays below soil; cotyledons/non-photosynthetic at early stage; monocots commonly show hypogeal germination (e.g., Coconut, Corn).
    • Epigeal germination: plumule emerges above soil; cotyledons become photosynthetic; many dicots show epigeal germination (e.g., Bean, Tamarind).

14.3 Reproduction of man

  • Puberty (adolescence): sexual maturity; secondary sexual characteristics begin to appear.
  • Secondary sexual characteristics (caused by hormones):
    • Males (testosterone): facial and body hair growth, broader shoulders, larynx deepens, body growth, testes begin producing sperm, genitals enlarge.
    • Females (estrogen and progesterone): pubic hair growth, breast development, widening of pelvic region, fat distribution, accelerated growth, onset of ovulation.
  • Process of reproduction: gamete production in reproductive systems.
  • Male reproductive system:
    • Main parts: Testes (testes in scrotum; contains ~250250 testicular lobules and ~10001000 seminiferous tubules); epididymis (sperm storage); vas deferens; seminal vesicles; prostate gland; Cowper's glands; penis; urethra; scrotum.
    • Function: production of sperm; storage and transport; semen = sperm + seminal fluid from glands; temperature regulation via scrotum to keep testes cooler than body temperature for healthy sperm; mature sperm have head, body, tail.
  • Female reproductive system:
    • Main parts: Ovaries (paired; cortex and medulla; follicles containing ova; graafian follicles, corpus luteum, corpus albicans); Fallopian tubes (oviducts) with fimbria; Uterus (fundus, body, cervix); Vagina (and endocervical canal).
    • Function: production of ova; transport to uterus via fallopian tubes; site of fertilization and embryo development; menstrual cycle regulation via ovaries and uterus.
  • Fertilization and embryology:
    • Menstrual cycle and ovulation release an ovum from the ovary into the fallopian tube.
    • Fertilization occurs when a sperm fuses with the ovum to form a zygote.
    • Zygote divides to form morula, which implants into the uterine wall (implantation).
    • Placenta and umbilical cord facilitate exchange of nutrients and oxygen between mother and fetus; blood does not normally cross via the umbilical cord.
  • Development of the fetus and birth:
    • Embryonic membranes and amniotic fluid surround the embryo; placenta forms a connection with the uterine wall; umbilical cord provides nutrient/oxygen exchange.
    • Parturition (childbirth): after ~280280 days of gestation, the fetus is delivered via the birth canal; placenta and membranes are expelled; umbilical cord is cut.
  • Hormonal coordination in reproduction: reproductive processes are regulated by hormones (FSH, LH, testosterone, estrogen, progesterone) secreted by endocrine glands.

14.4 Sexually transmitted diseases

  • Sexually transmitted diseases (STDs) are infections transmitted primarily by sexual contact and secretions; some blood transmission is possible.
  • Common STDs discussed:
    • Gonorrhoea: caused by Neisseria gonorrhoeae; symptoms include secretions and pain; can lead to infertility or blindness if untreated; curable with treatment.
    • Syphilis: caused by Treponema pallidum; initial painless blisters; later fever and throat pain; can spread to other organs if untreated; treatable if early.
    • Herpes: caused by Herpes simplex virus; painful blisters; no permanent cure.
    • AIDS: caused by HIV; may take years for symptoms to appear; fatal; transmitted by sexual contact and blood; avoid risk activities.

14.4 Hormonal coordination (summary)

  • Reproduction is regulated by hormones across sexes; key hormones include FSH, LH, testosterone, estrogen, and progesterone; produced by endocrine glands and act to control gamete production, maturation, ovulation, and pregnancy processes.

14.5 Summary (essential points)

  • Reproduction has two main modes: sexual (gametes, meiosis, genetic variation) and asexual (vegetative propagation, clones).
  • Plant reproduction involves vegetative propagation, flower structure, pollination (self vs cross), fertilization, fruit and seed formation, and seed germination including dormancy and its breakage.
  • Human reproduction includes puberty and secondary sexual characteristics, male and female reproductive systems, menstrual cycle (approx. 2828 days) with phases: follicular (≈ 1414 days), ovulation around day 1414, luteal (≈ 1414 days); fertilization leads to embryo development and pregnancy via the placenta and umbilical cord; parturition marks birth.
  • Hormonal coordination governs the reproductive processes in both sexes.
  • Sexually transmitted diseases (gonorrhoea, syphilis, herpes, AIDS) pose health risks and require responsible behavior and treatment where possible.