Plant Reproduction Chapter 30

The Plant Kingdom Nearly all are multicellular Eukaryotic Terrestrial Vast majority are photoautotrophs Energy from sun Carbon dioxide from air Minerals dissolved in water

Plant Kingdom Adaptations to land Well developed tissue Cuticle Stomata Protection of embryo Alternations of generations Sporophyte Diploid Produces spores by meiosis Reproductive structure

Plant Kingdom Gametophyte Haploid Produces gametes Egg and sperm Formed by mitosis Egg and sperm fuse to form zygote Undergoes mitosis Forms sporophyte

mitosis zygote (2n) multicelled sporophyte (2n) fertilizationmeiosis gametes (2n) spores (2n) multicelled gametophytes (n) mitosis Diploid Haploid Evolutionary Trend

sporophyte’s importance gametophyte’s importance green algaebryophytesfernsgymnospermsangiosperms zygote only, no sporophyte

Impacts, Issues Video Imperiled Sexual Partners

Plants and Pollinators Pollination Transfer of pollen to the stigma Pollen had evolved by 390 million years ago Sperm packed inside a nutritious package Transferred first by wind currents (abiotic) Later transferred by insects Plants that attracted insect pollinators with flowers had a reproductive advantage

Pollinators Pollination vectors Winds Insects Birds Other animals Coevolution with pollinators

Hummingbird drinking nectar of columbine flower

Long-nosed bat feeding on cactus flower at night

Pollinators Visual cues Size, shape, color, pattern Olfactory cues Odors from fruit or flowers Pollinators follow concentration gradient of volatile chemicals to their sources Reinforcements Nectar

Bee-attracting flower pattern Pollen Formation

Common dandelion under normal light Common dandelion under ultraviolet light

Angiosperm Life Cycle Plant life cycles are characterized by the alternation between a multicellular haploid (n) generation and a multicellular diploid (2n) generation Diploid sporophytes (2n) produce spores (n) by meiosis; these grow into haploid gametophytes (n) Gametophytes produce haploid gametes (n) by mitosis; fertilization of gametes produces a sporophyte

mature sporophyte male gametophyte female gametophyte DIPLOID HAPLOID fertilization meiosis (within anther) meiosis (within ovary) seed gametes (sperm) microspores megaspores gametes (eggs) (mitosis)

In angiosperms, the sporophyte is the dominant generation, the large plant that we see The gametophytes are reduced in size and depend on the sporophyte for nutrients The angiosperm life cycle is characterized by “three Fs”: Flowers Double Fertilization Fruits Angiosperm Life Cycle

Flower Structure and Function Flowers are the reproductive shoots of the angiosperm sporophyte they attach to a part of the stem called the receptacle Flowers consist of four floral organs: carpels, stamens, petals, and sepals

Flower Structure Nonfertile parts Sepals Receptacle Fertile parts Male stamens Female carpel (ovary) filamentantherstigmastyleovary receptacle sepal (all sepals combined are the flower’s calyx) OVULE (forms within ovary) petal (all petals combined are the flower’s corolla) STAMEN (male reproductive part) CARPEL (female reproductive part)

Kinds of Flowers Perfect flowers (Complete) Have both male and female parts Monoecious Imperfect flowers (Incomplete) Are either male or female Dioecious

Microspores to pollen Pollen Formation

Development of Male Gametophytes in Pollen Grains Pollen develops from haploid microspores within the microsporangia, or pollen sacs, of anthers Each microspore undergoes mitosis to produce two cells the generative cell the tube cell A pollen grain consists of the two-celled male gametophyte and the spore wall

Pollen Formation pollen sac anther filament microspore mother cell Meiosis pollen tube sperm nuclei mature male gametophyte stigma style of carpel Diploid Stage Haploid Stage microspores pollen grain Each anther has four pollen sacs Inside pollen sacs, cells undergo meiosis and cytoplasmic division to form microspores Microspores undergo mitosis to form pollen grains

Megaspores to eggs Egg Formation

Meiosis in ovule produces megaspores All megaspores but one disintegrate It undergoes mitosis three times without cytoplasmic division Result is a cell with eight nuclei Cytoplasmic division produces seven-celled female gametophyte 6 cells with 1 nucleus 1 being the egg 1 cell with 2 nuclei Dikaryotic Forms the endosperm

Pollination Transfer of pollen grains to a receptive stigma Pollen can be transferred by a variety of agents When a pollen grain lands on the stigma it germinates

Pollination

Double Fertilization Pollen

Double Fertilization After landing on a receptive stigma, a pollen grain produces a pollen tube that extends between the cells of the style toward the ovary It carries two sperm nuclei When pollen tube reaches an ovule, it penetrates embryo sac and deposits two sperm One sperm fertilizes the egg the other combines with the polar nuclei giving rise to the triploid endosperm (3n) nutritive tissue of the seed

Polar nuclei Pollen grain Ovule Two sperm Egg Stigma Pollen tube Style Ovary Micropyle Tube nucleus

Polar nuclei Polar nuclei Pollen grain Ovule Egg Synergid Two sperm Ovule Two sperm Egg Stigma Pollen tube Style Ovary Micropyle Tube nucleus

Zygote (2n) (egg plus sperm) Endosperm nucleus (3n) (two polar nuclei plus sperm) Polar nuclei Polar nuclei Pollen grain Ovule Egg Synergid Two sperm Ovule Two sperm Egg Stigma Pollen tube Style Ovary Micropyle Tube nucleus

Seed Formation Fertilization of the egg produces a diploid sporophyte zygote The zygote undergoes mitotic divisions to become an embryo sporophyte Seed A mature ovule encases an embryo sporophyte food reserves inside a protective coat

Structure of the Mature Seed The embryo and its food supply are enclosed by a hard, protective seed coat The seed enters a state of dormancy, wherein it stops growing and slows metabolism A mature seed is only about 5–15% water

Nourishing the Embryo Dicot embryo Absorbs nutrients from endosperm Stores them in its two cotyledons Monocot embryo Digestive enzymes are stockpiled in the single cotyledon Enzymes do not tap into the endosperm until the seed germinates

Eudicot seed development Nourishing the Embryo

Seed Dormancy: An Adaptation for Tough Times Seed dormancy increases the chances that germination will occur at a time and place most advantageous to the seedling The breaking of seed dormancy often requires environmental cues, such as temperature or lighting changes

Seed Germination / Seedling Development Germination depends on imbibition, the uptake of water due to low water potential of the dry seed The radicle (embryonic root) emerges first Next, the shoot tip breaks through the soil surface

Fruit A fruit develops from the ovary It protects the enclosed seeds and aids in seed dispersal

Animation: Fruit Development Right click slide / Select play

(b) Aggregate fruit Stamen (a) Simple fruit (c) Multiple fruit (d) Accessory fruit Stamen Ovary Stigma Ovary Stamen Ovary (in receptacle) Ovule Seed Pea flower Pea fruit Raspberry flower Carpels Pineapple inflorescence Raspberry fruit Carpel (fruitlet) Pineapple fruit Apple fruit Each segment develops from the carpel of one flower Flower Style Petal Sepal Apple flower Sepals Seed Remains of stamens and styles Receptacle

Seed Dispersal Fruit structure is adapted to mode of dispersal Some modes of seed dispersal: Wind currents Water currents Animals

wingseed (in carpel)

Dandelion “seeds” (actually one-seeded fruits) Dandelion fruit

Ant carrying seed with attached “food body”

Advantages and Disadvantages of Asexual vs Sexual Reproduction Asexual reproduction is also called vegetative reproduction Asexual reproduction can be beneficial to a successful plant in a stable environment However, a clone of plants is vulnerable to local extinction if there is an environmental change Sexual reproduction generates genetic variation that makes evolutionary adaptation possible

Artificial Propagation New plant develops from cuttings or fragments of shoot systems African violets and jade plants can be propagated from leaf cuttings Tissue-culture propagation Tiny plant bits are grown in rotating flasks containing a liquid growth medium