Reproduction and Development Notes PDF

CHAPTER 9: REPRODUCTION AND DEVELOPMENT 9.1: SEXUAL REPRODUCTION IN FLOWERING PLANTS DEVELOPMENT OF POLLEN GRAIN AND FORMATION OF DEVELOPMENT OF THE OVULE, EMBRYO SACS, AND THE MALE GAMETE. FORMATION OF THE FE...

CHAPTER 9: REPRODUCTION AND DEVELOPMENT 9.1: SEXUAL REPRODUCTION IN FLOWERING PLANTS DEVELOPMENT OF POLLEN GRAIN AND FORMATION OF DEVELOPMENT OF THE OVULE, EMBRYO SACS, AND THE MALE GAMETE. FORMATION OF THE FEMALE GAMETE Each ANTHER has four microsporangia / pollen sacs Each OVULE (within the ovary) has one megasporangium Each microsporangium/pollen sac contain a large number of Ovule’s megasporangium contain a large diploid cell diploid cell microsporocytes/microspore mother cells (2n) megasporocyte/megaspore mother cell (2n) Each microsporocyte/microspore mother cell divides by The megasporocyte/megaspore mother cell enlarges & divides by MEIOSIS to form four haploid cell microspores (n). MEIOSIS to form four haploid cell megaspores (n). *Four functional microspores refer to tetrad In many angiosperms, three megaspores would degenerate and only ONE megaspore survives. Each microspore divides by MITOSIS to form immature pollen The nucleus of surviving megaspore divides by MITOSIS 3 TIMES grain/male gametophyte that consists of 1 generative cell and 1 without cytokinesis to form an embryo sac / female gametophyte tube cell. with 8 haploid nuclei. During pollination, pollen grain is transferred from anther and Two groups of 4 nuclei move to both ends of the embryo sac. lands on a stigma 1 nucleus from each both ends move to the centre, forming polar The pollen grain/ male gametophyte becomes mature when the nuclei. Polar nuclei share the cytoplasm of the large central cell of the embryo sac generative cell divides by MITOSIS into two sperm cells (male 3 nuclei at the opposite end of micropyle / chalaza enclose by gametes). membrane become 3 antipodal cells (with unknown function) 3 nuclei located near the micropyle enclose by membrane: Tube cell will produce a pollen tube, which delivers male gamete One nucleus becomes egg cell to the egg. Two nuclei become synergid cells (flank the egg cell and help attract and guide the pollen tube to the egg cell in embryo sac)  CHAPTER 9: REPRODUCTION AND DEVELOPMENT Similarity and Differences Between Embryo Sac and Pollen Grain Similarities of embryo sac and pollen grain; Both are gametophyte. Both are produced through mitosis of haploid spores. Double Fertilization Definition of Double Fertilization: -Process of one haploid sperm cell (n) fertilizes with an egg cell (n) to form diploid zygote (2n) -and the other haploid sperm cell (n) fuses with two polar nuclei to form triploid endosperm (3n). *The union of two male gametes (sperm cells) with different nuclei of the embryo sac is called double fertilization *The zygote (2n) develops into growing embryo *The endosperm (3n) become food storing tissue that nourishes (provide nutrient) the growing embryo Process of Double Fertilization: -After pollen grain landing on a stigma (pollination), pollen grain absorbs moisture (water) on stigma and germinates - Tube cell produce a pollen tube that elongate through the style towards the ovary. - The generative cell divides by mitosis to produce two male gametes/sperm cells -In response to chemical attraction from synergid cells, the tip of the pollen tube grows toward the micropyle. -Nucleus tube cell degenerate -Pollen tube discharges two male gametes/2 sperm cells into the embryo sac through micropyle -One haploid sperm cell (n) fertilizes with an egg cell (n) to form diploid zygote (2n). Zygote then develops into embryo -and the other haploid sperm cell (n) fuse with two polar nuclei to form triploid endosperm (3n). Endosperm become food storing tissue that nourish growing embryo Product after double fertilization: - Ovule / embryo sac develops into a seed. - Zygote (2n) develops into an embryo. Embryo later develops into plumule (first shoot) and radicle (first root) - Integuments become testa (seed coat) that enclose the seed - Ovary develops into a fruit which protects the seed  CHAPTER 9: REPRODUCTION AND DEVELOPMENT 9.2: HUMAN REPRODUCTIVE SYSTEM (MALE) Male reproductive organ (testes) and the structure of spermatozoa Male reproductive system consists of: -Testes (singular: testis) -A system of ducts (including epididymis, Vas deferens, ejaculatory ducts, and urethra) -Accessory sex glands (seminal vesicles, prostate, bulbourethral glands) -Supporting structure (scrotum, penis) Testes: -Located in the scrotum -Each testis consist of many long, highly coiled tubules called seminiferous tubules where sperms are produced. -Between each seminiferous tubule are Leydig cells / Interstitial cells -Sperm from the seminiferous tubules pass into the epididymis. -During ejaculation, sperm are pass from epididymis into Vas deferens. Structure of spermatozoa @ sperm cell: consist of head, neck, middle piece and tail  CHAPTER 9: REPRODUCTION AND DEVELOPMENT Spermatogenesis: -The series of cell divisions in the testes that result in the production and development of spermatozoa (mature sperm cell) Process of spermatogenesis: (occur in seminiferous tubules of testes; continuous process in adult male) 1. In MATURE TESTES, spermatogonial stem cell (2n) divides by mitosis to form diploid spermatogonia (2n). 2. Spermatogonium (2n) divides by mitosis to form more spermatogonia (2n). Some spermatogonia (2n) remains at the basal membrane. 3. Some diploid spermatogonia (2n) enlarge, undergo mitosis and differentiate to form diploid primary spermatocyte (2n). 4. Each diploid primary spermatocyte (2n) undergo meiosis I forming 2 haploid secondary spermatocytes (n) 5. Haploid secondary spermatocyte (n) then undergo meiosis II forming 4 haploid spermatids (n) 6. As the spermatocytes develop, they are being pushed towards the lumen. 7. Spermatids (n) are non-motile and become embedded in Sertoli cells (Sertoli cells provide nutrients to the spermatids) 8. Haploid spermatid (n) undergo differentiation (spermiogenesis) to form 4 haploid spermatozoa/sperm cell (n). 9. Haploid spermatozoa/sperm cell (n) release into the lumen of seminiferous tubule *SPERMIOGENESIS (differentiation and maturation of spermatids to form spermatozoa) - spermatid elongate, shed excess cytoplasm, developing acrosome and form a tail.  CHAPTER 9: REPRODUCTION AND DEVELOPMENT Hormonal control in spermatogenesis: At PUBERTY, hypothalamus secrete Gonadotropin- Releasing Hormone (GnRH) GnRH stimulate the anterior pituitary gland to secrete Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH). LH acts on and stimulate Leydig cells / Interstitial cells in the testes Leydig cells secrete testosterone. Testosterone promotes spermatogenesis in the tubules and acts on Sertoli cells by increasing its responsiveness to FSH FSH acts on and stimulate Sertoli cells. Sertoli cells nourish developing sperms/spermatids and facilitate spermiogenesis (development of spermatid to spermatozoa by providing nutrients to spermatid) Sertoli cells also synthesis Androgen Binding Protein (ABP) which bind to testosterone and promote spermatogenesis. 2 negative feedback mechanism control sex hormone production / maintain androgen levels in the normal range in male: Sertoli cells secrete inhibin which inhibit anterior pituitary gland to reduce FSH secretion by negative feedback mechanism Testosterone regulate blood level of GnRH, FSH and LH by inhibit both hypothalamus and anterior pituitary gland by negative feedback mechanism Testosterone inhibit hypothalamus to reduce GnRH secretion. Testosterone also inhibit anterior pituitary gland to reduce FSH and LH secretion. Inhibin and testosterone involve in negative feedback mechanism to control spermatogenesis. Negative feedback mechanism that regulates the secretion of GnRH, LH and FSH Negative feedback of testosterone on hypothalamus and anterior pituitary gland regulates levels of GnRH, FSH and LH. - In HIGH LEVELS of testosterone: Testosterone acts on and inhibit hypothalamus to reduce GnRH secretion. Testosterone also acts on and inhibit anterior pituitary gland to block the normal actions of GnRH Therefore, decrease in GnRH will reduce FSH and LH secretion by the anterior pituitary gland. FSH secretion also subject to negative feedback. Once RATE SPERMATOGENESIS IS HIGHER THAN NORMAL, Sertoli cells secrete inhibin (peptide hormone) Inhibin acts on and inhibit anterior pituitary gland to reduce FSH secretion. These two negative feedback mechanisms maintain androgen levels in the normal range in male  CHAPTER 9: REPRODUCTION AND DEVELOPMENT 9.2: HUMAN REPRODUCTIVE SYSTEM (FEMALE) Structure of Secondary Oocyte: - Contain haploid nucleus (n) within cytoplasm that arrest at metaphase II of meiosis II. - Plasma membrane of secondary oocyte surrounded by zona pellucida (thick glycoprotein layer) together with first polar body (n) Zona pellucida develops as a jelly coat First polar body (n) produced during the completion of first meiotic division of secondary oocyte and usually Secondary Oocyte: degenerate - A female gamete which produces within the ovary. - The zona pellucida is surrounded with layers of - It is an immature / unfertilized egg (n), arrested at follicle/granulosa cells called corona radiata. metaphase II of meiosis II. Corona radiata supply nutrients to the developing oocyte - Become ovum (mature / fertilized egg) after meiosis II is - During ovulation, secondary oocyte is released through the completed ovary wall into cavity of Fallopian tube/ oviduct.  CHAPTER 9: REPRODUCTION AND DEVELOPMENT Oogenesis: The production and growth of ovum (egg cells) that takes place in the ovary (and oviduct) -Oogenesis is discontinuous process because it: 1. begins before birth but arrested at prophase I. 2. resumed under hormonal control at puberty. 3. completed if penetrated by sperm. Follicle: A structure containing developing oocyte. Secrete estrogen Corpus luteum: Remaining follicular tissue after ovulation. Secrete progesterone and estrogen Process of Oogenesis: -IN FEMALE EMBRYO, primordial germ cell (2n) divides by mitosis repeatedly into oogonium (2n) -Each oogonium (2n) divides by mitosis forming primary oocyte (2n). Primary oocyte resides within small/primordial follicle. -Primary oocyte (2n) undergoes meiosis I, BUT arrested at prophase I. -Females baby is born with about 1-2 million primary oocytes within small follicles in their ovaries (but only about 500 fully mature between puberty and menopause). No further development occurs in childhood until puberty. - AT PUBERTY, anterior pituitary gland secretes Follicle- Stimulating hormone (FSH) to stimulate the development a small number of follicles in the ovary. - Typically, only one follicle fully matures each month. -Primary oocyte within follicle completes meiosis I by unequal cytokinesis to produce first polar body (n) (eventually degenerates) and secondary oocyte (n). -Secondary oocyte (n) undergoes meiosis II but arrested at metaphase II. -DURING OVULATION, mature follicle (Graafian follicle) is ruptured and secondary oocyte (n) is released from the ovary into the Fallopian tube. - Ruptured follicle left behind develops into corpus luteum - Corpus luteum secrete progesterone and estrogen that help stimulate to maintain thickness of endometrium during pregnancy. -IF A SPERM (n) PENETRATE THE SECONDARY OOCYTE (n), secondary oocyte completes meiosis II by unequal cytokinesis -Produces second polar body (n) (eventually degenerate) and ovum (n) – single mature fertilized egg cell containing sperm head. *If no sperm penetrate the secondary oocyte: - Secondary oocyte not complete its meiosis II (degenerate) - Corpus luteum degenerate - A new follicle matures for next cycle.  CHAPTER 9: REPRODUCTION AND DEVELOPMENT Spermatogenesis Oogenesis Occurs in the seminiferous Occurs in ovary tubule of the testes One diploid primary One diploid primary oocyte spermatocyte undergoes undergoes meiosis with meiosis with equal unequal cytokinesis to form cytokinesis to form four one large haploid egg cell haploid spermatozoa/sperm (ovum) and three small cells haploid polar bodies (degenerate) Occurs throughout Occurs in the female embryo adolescence and adulthood, before birth, and continues once puberty started during puberty until menopause Produce mature sperm cells Produce mature egg cell in a in a continuous sequence discontinuous sequence with a long interruption period.  CHAPTER 9: REPRODUCTION AND DEVELOPMENT Ovarian Cycle PHASE 1: Follicular phase (start from day 0- 14) Begins when hypothalamus secrete Gonadotropin-Releasing Hormone (GnRH) which stimulate the anterior pituitary gland to secrete small amounts of Follicle-stimulating hormone (FSH) & Luteinizing hormone (LH) FSH stimulate follicles development and growth (aided by LH). Several follicles begin to grow but only one follicle matures. The cells of the growing follicles start to secrete estrogen, leads to a slow rise in estrogen level. The low level of estrogen inhibits anterior pituitary gland; keeping the levels of FSH & LH relatively low. Then, secretion of estrogen by growing follicle increase sharply. High level of estrogen stimulates hypothalamus to increase secretion of GnRH High level of GnRH stimulates anterior pituitary gland to increase secretion of FSH and LH, making levels FSH and LH increase markedly. Large amounts of LH secreted is called LH surge. LH surge due to increases GnRH sensitivity of LH-releasing cells in the anterior pituitary gland. High level of LH stimulates final maturation of the follicle. Ovulation (day 14) – about a day after LH surge High level (peak) of LH stimulates ovulation. The maturing follicle and adjacent wall of the ovary rupture, releasing the secondary oocyte into Fallopian tube. Only one secondary oocyte is released each month by one of the ovaries. PHASE 2: Luteal phase (days 15 – 28) LH stimulate remaining follicular tissue of ruptured follicle to form corpus luteum. Under continue influence of LH, corpus luteum secretes progesterone and estrogen High concentration of progesterone and estrogen exert negative feedback and inhibit hypothalamus and anterior pituitary gland. Thus, reducing secretion of GnRH, LH & FSH to very low levels. Prevent growth and maturation of another follicles when pregnancy may be under way IF PREGNANCY DO NOT OCCURS near the end of Luteal Phase, corpus luteum degenerate due to the low level of LH Causing sharp decline in concentration of both estrogen and progesterone. Anterior pituitary gland begins to secrete enough FSH to stimulate the growth of new follicles, Initiating the NEXT OVARIAN CYCLE. Uterine / Menstrual Cycle Phase 1: Menstrual Flow Phase (days 1 – 5) begin when rapid drop in level of estrogen and progesterone since the corpus luteum has degenerated. Lead to constriction of arteries in the endometrium, thus lower the supply of blood and oxygen. Cells of the endometrium die and disintegrate. Blood and endometrial tissue are shed and discharged through menstruation. As endometrium is shedding, hypothalamus secrete GnRH. GnRH stimulate anterior pituitary gland to secrete FSH and LH for development and growth of follicles in ovary. Growing follicles secrete estrogen. Phase 2: Proliferative phase (days 6 – 14) Increasing amount of estrogen secreted by growing follicles stimulate repair of endometrium and stimulate the endometrium to thicken. Endometrium is supplied with a network of blood vessels. Phase 3: Secretory phase (days 15 – 28) As corpus luteum secrete progesterone and estrogen Stimulate to maintain the thickness of endometrium Progesterone and estrogen stimulate arteries enlarge and endometrial glands grows. Endometrial glands ready to secrete nutrient that can sustain early embryo Ready for implantation of embryo - IF PREGNANCY OR EMBRYO IMPLANTATION DO NOT OCCUR near the end of Secretory Phase, corpus luteum degenerated causing estrogen and progesterone level decline. - The endometrium shed again and menstruation begin (Menstrual Flow Phase OR new uterine cycle begins) CHAPTER 9: REPRODUCTION AND DEVELOPMENT  CHAPTER 9: REPRODUCTION AND DEVELOPMENT IF PREGNANCY OR EMBRYO IMPLANTATION OCCURS: New ovarian cycle and uterine / menstrual cycle stop Developing embryo secrete hormone human chorionic gonadotropin (hCG). Stimulate to maintain the corpus luteum until placenta is fully developed. since corpus luteum secrete both progesterone and estrogen to maintain thickness of endometrium (maintain pregnancy) When placenta is fully developed, corpus luteum also degenerate as placenta begins to secrete its own estrogen and progesterone  CHAPTER 9: REPRODUCTION AND DEVELOPMENT 9.3: FERTILIZATION AND FOETAL DEVELOPMENT Fertilization: The fusion of haploid nuclei of the male gamete (sperm) and female gamete (egg) to produce a diploid zygote. Occurrence site: One third of the Fallopian tube. Genetic consequences of fertilization: – provide genetic variation – determination of sex of offspring Stages that lead to fertilization: Stage 1: Capacitation Functional maturation process of spermatozoa / sperm through secretions in female reproductive tract to enhance sperm motility Due to secretions from female reproductive tract, certain molecules on the sperm surface is altered: -The removal of cholesterol, glycoprotein and protein from acrosomal cap → preparation for acrosomal reaction The capacitated sperm able to migrate / move through layers of follicle cells (corona radiata) before it reaches zona pellucida. Stage 2: Acrosomal reaction As capacitated sperm move through the layers of follicle cells (corona radiata) and reach the zona pellucida, it triggers exocytosis of sperm’s acrosome. Sperm head bind to the specific receptor (ZP3 receptor) on zona pellucida The acrosome membrane will rupture/disperse Acrosome release hydrolytic enzymes (hyaluronidase and acrosin/protease) into zona pellucida by exocytosis → to digest/hydrolyze the zona pellucida - To enable the penetration of sperm through digested zona pellucida into plasma membrane of secondary oocyte and entry of sperm. Stage 3: Fusion of sperm head membrane and secondary oocyte Sperm’s head reach the plasma membrane of secondary oocyte Fusion of sperm head membrane and secondary oocyte membrane; triggers cortical reaction. Stage 4: Cortical reaction As sperm head membrane fuse with secondary oocyte Cortical granule fuse with egg’s plasma membrane Cortical granule release enzyme into zona pellucida by exocytosis -To harden the zona pellucida -Also to destroy/alter sperm receptors on zona pellucida -Changes in zona pellucida prevent polyspermy (prevent other sperm from entering the egg) // act as slow block polyspermy. Sperm nucleus enters cytoplasm of the secondary oocyte Stimulate secondary oocyte to complete meiosis II forming ovum Pronuclei of sperm and ovum fuse, form diploid zygote (2n).  CHAPTER 9: REPRODUCTION AND DEVELOPMENT Foetal Development Embryogenesis: The process by which the embryo form and develop from zygote to the formation of morula, blastocyst and gastrula. Cleavage: A series of continuous rapid mitotic division without significant growth during early development at oviduct. Development from zygote into blastocyst: – Increase the cell number. – The size of cell does not increase No growth period. Each cell is called as blastomere. Process cleavage: Initially zygote divide forming two-celled embryo. Repeated division forms four-celled and continue dividing forming 32-cells called morula. – Morula: A solid sphere ball of cells (still surrounded by zona pellucida). – Floats in the uterine cavity by several days and nourished by endometrial secretions. Cell continues divide forming 64 to several hundreds of cells forming blastocyst. – Blastocyst: A hollow sphere ball of cells with fluid-filled cavity (blastocoel) with inner cell mass and trophoblast layer. Then blastocyst will be implanted in the endometrium @ uterine lining, occur one week after fertilization – Implantation of embryo initiated by the enzyme that secreted from trophoblast, breakdown the endometrium After implantation, trophoblast cell of blastocyst will start secreting hCG (human chorionic gonadotropin) hormone. hCG stimulate corpus luteum to maintain/not degenerate so that corpus luteum continue secrete the progesterone and estrogen. It also cause ‘morning sickness" in some women. Pregnancy test measures the hCG amount in urine & blood Embryo get nutrients direct from the endometrium for 2-4 weeks until the placenta is formed.  CHAPTER 9: REPRODUCTION AND DEVELOPMENT Gastrulation A series of cells and tissue movements in which the blastocyst fold inwards, producing a three-germ layered embryo, the gastrula. Gastrula: Organogenesis: Process of body organ formation. Organs arise from three primary germ layers ectoderm, mesoderm and endoderm. – Embryo now called as fetus Neurulation is the initial stage of organogenesis – It is a process of neural tube formation.  CHAPTER 9: REPRODUCTION AND DEVELOPMENT 9.4: ROLES OF HORMONES DURING PREGNANCY AND PARTURITION During Pregnancy: During first trimester (1-3 month): - After implantation, the trophoblast cells of implanted blastocyst start to secrete hCG (Human chorionic gonadotropin) - hCG stimulate to maintain the corpus luteum (prevent corpus luteum from degenerate) - hCG stimulate the corpus luteum to continue secretion of progesterone and estrogen. - hCG level increases during the first trimester. - Estrogen and progesterone: stimulate to maintain the thickness of endometrium for embryonic development. - Estrogen and progesterone: stimulate to maintain pregnancy. - Estrogen stimulate development of uterine wall (including muscle need to expel fetus during delivery) - High level of progesterone: maintain pregnancy -High level of progesterone: inhibit oxytocin (to prevent uterine contraction) -High level of progesterone: inhibit prolactin and oxytocin (to prevent lactation until after birth) -High level of progesterone bring rapid changes in the mother at the end of first trimester: mucus in the cervix forms plug that protect against infection maternal part of placenta grows mother’s breast and uterus get larger prevent development of new follicle and ovulation prevent menstrual cycle. - End of first trimester, corpus luteum starts to degenerate. - Placenta is develop and take over the roles of corpus luteum to secrete progesterone and estrogen. In the middle of pregnancy (second trimester) (4-6 month): -Progesterone and estrogen level stabilize as hCG secretion decline and corpus luteum degenerate. -Placenta completely take over the secretion of progesterone and estrogen. Final week of pregnancy (end of third trimester): - Estrogen level reaches its highest level, stimulate formation of oxytocin receptors on uterus -Progesterone level decrease, stimulate beginning of uterine contraction that will lead to birth.  CHAPTER 9: REPRODUCTION AND DEVELOPMENT During Parturition: During final week of pregnancy: ⚫ Estrogen reaches its highest level - Placenta stimulate synthesis of From DHEAS from adrenal gland. DHEAS convert to high level of estrogen placenta ❖ High level of estrogen stimulates formation of oxytocin receptors on uterus ⚫ Decrease in progesterone receptor on uterus causes progesterone level decrease/drops off ❖ Stimulate beginning of uterine contraction that will lead to birth. ⚫ Oxytocin released by the fetus & mother's posterior pituitary gland ❖ Stimulate uterus contraction ⚫ Oxytocin also stimulate placenta to secrete prostaglandins ❖ Stimulate more contractions of uterus ⚫ Both oxytocin and prostaglandins induce and regulate contractions of uterus ⚫ Uterine contraction stimulate secretion of more oxytocin and prostaglandins ❖ Which in turn stimulates further contractions of uterus by positive feedback Uterus contraction occurs until baby are deliver out through cervix 9.5: GROWTH PATTERNS IN HUMAN AND PLANTS Growth: Any permanent and irreversible increase in quantitative parameters (such as size/ dry mass/ volume/ length/ height/ surface area) of an organism against time. Beginning from the zygote to adult stage  CHAPTER 9: REPRODUCTION AND DEVELOPMENT 1. Human Growth Curve -Growth in human refers to an increase in mass/height - Shows a modified sigmoid curve -Human has two main rapid growth @ growth spurts: i. Infant phase ii. Adolescent phase - Between these 2 phases, there is a period of a relatively slow growth Phase 1. Infant phase: Rapid growth rate (irrespective in both male or female babies) Phase 2. Childhood phase: Slower growth rate Beginning at the age of four, the rate is slightly higher in boys than in girls Phase 3. Adolescent phase: Rapid growth rate In the earlier part of this growth phase, females demonstrate a more rapid growth rate ✔i.e. two years earlier than begins in males ✔At the later part of this growth phase, the male growth rate becomes higher than the female growth rate. ✔This difference results in the females attaining puberty at an earlier age (around age 12) compared to males (around age 14) Phase 4. Adult phase: Growth rate is zero Most individuals would have attained maturity Males achieve this phase at the age about 18 wile females at the age around 16. Phase 5. Ageing / senescence phase: Growth rate is negative The body proportion will start to decrease starting from the age of 30.  CHAPTER 9: REPRODUCTION AND DEVELOPMENT 2.Allometric Growth (human organs) The organs grow at different rates compared to other body parts and overall growth The increases in size is accompanied by the changes in shape The relative sizes of organs are different from the final size -Head grows rapidly in the first 5 years after birth (for development of brain) -Growth rate of lymphoid tissues is very rapid in childhood phase - because lymphoid tissues are required by young individual to defense against diseases, as immunity has not yet been acquired. -Reproductive organs grows very little in early life but rapidly during adolescent phase (at puberty) 3.Limited Growth (Annual Plants) Growth of an organism to a maximum size Growth is complete when the organism achieves its maximum size at maturity Then, the organism stop growing and demonstrates negative growth or senescence / ageing Growth curve is shown by single sigmoid curve Demonstrated by annual plants such as pea plant, corn, watermelon, sunflower Annual plant: plant with relatively short life span, complete its life cycle from germinate until disperse seed only in one year Shows small reduction in dry mass during initial growth - due to seed germination; food stored is used before the first leaves undergo photosynthesis Loss of dry mass is replaced when leaves develop and photosynthesis occur Then, growth rate is very rapid, until it becomes constant at later stage (achieve maturity) Growth rate gradually decrease due to senescence / ageing  CHAPTER 9: REPRODUCTION AND DEVELOPMENT 4.Unlimited Growth (Perennial Plants) Growth of an organism shows continuous grow @ non- stop growth throughout its life - except restricted by environmental factors such as parasitic infection, disease, natural disaster) Growth curve consists of a series of smaller sigmoid curve - Each curve represents the growth in one year. Demonstrated by perennial plants such as large woody tree - perennial plant: plant with relatively long life span: life more than one year