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Chapter 38: Angiosperm Reproduction
1. The life cycle of an angiosperm….
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Figure 38.2 An overview of angiosperm reproduction
Filament Anther Stamen Anther at tip of stamen Pollen tube Germinated pollen grain (n) (male gametophyte) on stigma of carpel Ovary (base of carpel) Ovule Embryo sac (n) (female gametophyte) FERTILIZATION Egg (n) Sperm (n) Petal Receptacle Sepal Stigma Style Ovary Key Haploid (n) Diploid (2n) (a) An idealized flower. (a) Simplified angiosperm life cycle. See Figure for a more detailed version of the life cycle, including meiosis. Mature sporophyte plant (2n) with flowers Seed (develops from ovule) Zygote (2n) Embryo (2n) (sporophyte) Simple fruit (develops from ovary) Germinating seed Carpel
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Chapter 38: Angiosperm Reproduction & Biotechnology
Let’s review the life cycle of an angiosperm…. Can all plants self-fertilize? No Complete – have all 4 floral parts – sepal, petal, carpel, stamen Incomplete – lack 1 or more parts Perfect flowers – have both stamen & carpels Imperfect flowers – missing either stamen or carpels Stamenate – have stamen Carpellate – have carpel Monoecious: have stamenate and carpellate flowers on same plant Dioecious: have stamenate and carpellate flowers on different plants How does pollen (male gametophyte) & the embryo sac (female gametophyte) develop?
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Figure 38.4 The development of angiosperm gametophytes (pollen grains and embryo sacs)
Development of a male gametophyte (pollen grain). Pollen grains develop within the microsporangia (pollen sacs) of anthers at the tips of the stamens. (b) Development of a female gametophyte (embryo sac). The embryo sac develops within an ovule, itself enclosed by the ovary at the base of a carpel. Pollen sac (microsporangium) Each one of the microsporangia contains diploid microsporocytes (microspore mother cells). 1 Mega- sporangium 1 Within the ovule’s megasporangium is a large diploid cell called the megasporocyte (megaspore mother cell). Micro- sporocyte Ovule Mega- sporocyte MEIOSIS Integuments Each microsporo- cyte divides by meiosis to produce four haploid microspores, each of which develops into a pollen grain. 2 Micro- Spores (4) Micropyle 2 The megasporo- cyte divides by meiosis and gives rise to four haploid cells, but in most species only one of these survives as the megaspore. Surviving megaspore Each of 4 microspores Female gametophyte (embryo sac) MITOSIS Ovule Antipodel Cells (3) Generative cell (well form 2 sperm) Male Gametophyte (pollen grain) 3 A pollen grain becomes a mature male gametophyte when its generative nucleus divides and forms two sperm. This usually occurs after a pollen grain lands on the stigma of a carpel and the pollen tube begins to grow. (See Figure 38.2b.) Polar Nuclei (2) 3 Three mitotic divisions of the megaspore form the embryo sac, a multicellular female gametophyte. The ovule now consists of the embryo sac along with the surrounding integuments (protective tissue). Egg (1) Nucleus of tube cell Integuments Synergids (2) 20 m Key To labels Ragweed Pollen grain Embryo sac 75 m Haploid (n) Diploid (2n) 100 m 100 m Diploid (2n)
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Chapter 38: Angiosperm Reproduction & Biotechnology
Let’s review the life cycle of an angiosperm…. Can all plants self-fertilize? How does pollen (male gametophyte) & the embryo sac (female gametophyte) develop? How can plants prevent self-fertilization? Genetic adaptations – “S genes” – self-incompatibility – reject self Anatomical adaptations – pin & thrum flowers Temporal adaptations – male & female parts mature at different times Stigma Anther With pollen Pin flower Thrum flower
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Chapter 38: Angiosperm Reproduction & Biotechnology
Let’s review the life cycle of an angiosperm…. Can all plants self-fertilize? How does pollen (male gametophyte) & the embryo sac (female gametophyte) develop? How can plants prevent self-fertilization? How does double fertilization occur & why is this important?
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Figure 38.6 Growth of the pollen tube and double fertilization
If a pollen grain germinates, a pollen tube grows down the style toward the ovary. Stigma The pollen tube discharges two sperm into the female gametophyte (embryo sac) within an ovule. One sperm fertilizes the egg, forming the zygote. The other sperm combines with the two polar nuclei of the embryo sac’s large central cell, forming a triploid cell that develops into the nutritive tissue called endosperm. 1 2 3 Polar nuclei Egg Pollen grain Pollen tube 2 sperm Style Ovary Ovule (containing female Gametophyte, or Embryo sac) Micropyle Ovule Polar nuclei Two sperm about to be discharged Endosperm nucleus (3n) (2 polar nuclei plus sperm) Zygote (2n) (egg plus sperm)
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Chapter 38: Angiosperm Reproduction & Biotechnology
Let’s review the life cycle of an angiosperm…. Can all plants self-fertilize? How does pollen (male gametophyte) & the embryo sac (female gametophyte) develop? How can plants prevent self-fertilization? How does double fertilization occur & why is this important? What happens after fertilization? - Development of an embryo
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Figure 38.7 The development of a eudicot plant embryo
Ovule Terminal cell Endosperm nucleus Basal cell Zygote Integuments Proembryo Cotyledons Shoot apex Root Seed coat Suspensor Suspensor Suspensor
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Figure 38.8 Seed structure Seed coat Epicotyl Hypocotyl Radicle
(a) Common garden bean, a eudicot with thick cotyledons. The fleshy cotyledons store food absorbed from the endosperm before the seed germinates. (b) Castor bean, a eudicot with thin cotyledons. The narrow, membranous cotyledons (shown in edge and flat views) absorb food from the endosperm when the seed germinates. (c) Maize, a monocot. Like all monocots, maize has only one cotyledon. Maize and other grasses have a large cotyledon called a scutellum. The rudimentary shoot is sheathed in a structure called the coleoptile, and the coleorhiza covers the young root. Seed coat Radicle Epicotyl Hypocotyl Cotyledons Endosperm Scutellum (cotyledon) Coleoptile Coleorhiza Pericarp fused with seed coat
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Chapter 38: Angiosperm Reproduction & Biotechnology
Let’s review the life cycle of an angiosperm…. Can all plants self-fertilize? How does pollen (male gametophyte) & the embryo sac (female gametophyte) develop? How can plants prevent self-fertilization? How does double fertilization occur & why is this important? What happens after fertilization? How do fruits develop?
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Figure 38.9 Developmental origin of fruits
Simple fruit. A simple fruit develops from a single carpel (or several fused carpels) of one flower (examples: pea, lemon, peanut). (a) Aggregate fruit. An aggregate fruit develops from many separate carpels of one flower (examples: raspberry, blackberry, strawberry). (b) Multiple fruit. A multiple fruit develops from many carpels of many flowers (examples: pineapple, fig). (c) Pineapple fruit Raspberry fruit Pea fruit Stamen Carpel (fruitlet) Stigma Ovary Raspberry flower Each segment develops from the carpel of one flower Pineapple inflorescence Carpels Flower Ovule Pea flower Seed
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Chapter 38: Angiosperm Reproduction & Biotechnology
Let’s review the life cycle of an angiosperm…. Can all plants self-fertilize? How does pollen (male gametophyte) & the embryo sac (female gametophyte) develop? How can plants prevent self-fertilization? How does double fertilization occur & why is this important? What happens after fertilization? How do fruits develop? What happens during dormancy? Germination? --Seed remains dormant until germination is activated by… -heat -light -water -extended cold weather -animal’s digestive juices
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Figure 39.11 Gibberellins mobilize nutrients during the germination of grain seeds
2 The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA -amylase Radicle Sugar 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling. 2
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Figure 38.10 Two common types of seed germination
Foliage leaves Cotyledon Hypocotyl Radicle Epicotyl Seed coat Common garden bean. In common garden beans, straightening of a hook in the hypocotyl pulls the cotyledons from the soil. (a) Coleoptile Maize. In maize and other grasses, the shoot grows straight Up through the tube of the coleoptile. (b)
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