Plant Reproduction and Development

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Chapter 38

• Plant Reproduction and Development

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Alternation of Generations

• Angiosperms and other plants exhibit alternation
  of generations haploid (n) and diploid (2n)
  generations take turns producing each other
• Sporophyte diploid plant that produces haploid
  spores by meiosis
• Gametophyte haploid plant that produces gametes

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Alternation of Generations

• Fertilization results in diploid zygotes, which
  divide by mitosis and form new sporophytes
• Sporophyte dominant in angiosperms
• Evolutionary history has reduced gametophytes in
  angiosperms to only a few cells, not an entire
  plant

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Flowers

• Angiosperm sporophytes produce unique
  reproductive structures called flowers
• Flowers consist of four types of highly modified
  leaves
• Sepals
• Petals
• Stamen
• Pistil (or carpel)
• Their site of attachment to the stem is the
  receptacle

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Flower Structure
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Flower Anatomy

• Sepals and petals are nonreproductive organs
• Sepals protect the other three, the floral bud
• Petals attract pollinators and act as landing
  pads

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Flower Anatomy

• Stamen and carpels are male and female
  reproductive organs, respectively
• Stamen consists of filament (long, thin) and
  anther (pollen)
• Carpel consists of stigma (sticky opening),
  style (long tube connecting stigma to ovary),
  ovary (houses ovules becomes fruit), and ovules
  (develops female gametes become seeds)

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Flower Anatomy

• Complete flowers have all four floral organs
• Ex Trillium
• Incomplete flowers missing one or more of the
  four floral organs

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Flower Anatomy

• Bisexual flower (perfect flower) is equipped with
  both stamens and carpals
• All complete and many incomplete flowers are
  bisexual
• A unisexual flower is missing either stamens
  (carpellate flower) or carpels (staminate flower)

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Unisexual Flowers

• Monoecious plants staminate and carpellate
  flowers at separate locations on the same
  individual plant
• Ex corn ears derived from clusters of carpellate
  flowers tassels consist of staminate flowers

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Unisexual Flowers

• Dioecious plants staminate and carpellate
  flowers on separate plants
• Ex Date palms and Sagittaria (below) have
  carpellate individuals that produce dates and
  staminate individuals that produce pollen

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Gamete Formation

• Development of angiosperm gametophytes involves
  meiosisand mitosis

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Gamete Formation

• The male gametophytes are sperm-producing
  structures called pollen grains, which form
  within the pollen sacs of anthers
• The female gametophytes are egg-producing
  structures called embryo sacs, which form within
  the ovules in ovaries

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Male Gamete Formation

• The male gametophyte begins development within
  the sporangia (pollen sacs) of the anther
• Within the sporangia are microsporocytes, each of
  which will from four haploid
  microspores through meiosis
• Each microspore can eventually give rise to a
  haploid male gametophyte

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Male Gamete Formation

• A microspore divides once by mitosis and produces
  a generative cell and a tube cell
• Generative cell will eventually form sperm
• Tube cell, enclosing the generative
  cell, produces the pollen tube delivers
  sperm to egg

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Male Gamete Formation

• This two-celled structure (generative and tube
  cells) is encased in a thick, ornate,
  distinctive, and resistant wall a pollen grain
  an immature male gametophyte

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Female Gamete Formation

• Ovules, each containing a single sporangium, form
  within the chambers of the ovary
• One cell in the sporangium of each ovule, the
  megasporocyte, grows and then goes through
  meiosis, producing four haploid megaspores
• In many angiosperms, only one megaspore survives

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Female Gamete Formation

• This megaspore divides by mitosis three times,
  resulting in one cell with eight haploid nuclei
• Membranes partition this mass into a
  multicellular female gametophyte the egg
  sac

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Female Gamete Formation

• At one end of the egg sac, two synergid cells
  flank the egg cell
• Synergids attract and guide the pollen tube
  formation
• At the other end of the egg sac are three
  antipodal cells no idea what they do

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Female Gamete Formation

• The other two nuclei, the polar nuclei, share the
  cytoplasm of the large central cell of the embryo
  sac
• The ovule now consists of the embryo sac
  and the surrounding integuments (from the
  sporophyte)

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Angiosperm Pollination

• The successful transfer of pollen from anther to
  stigma
• NOT fertilization fusion of gametes
• Pollination leads to fertilization
• Cross-pollination vs. self-pollination
• Most angiosperms are pollinated by insects,
  birds, and mammals (vectors) that reward the
  species with food in the form of nectar
• Some are pollinated by wind (corn, wheat) and
  have small, plain, non-fragrant flowers

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Angiosperm Pollination

• Fragrance, pattern, and colors are designed to
  attract the vector so it will pick up pollen and
  bring it to the next flower
• Some vectors get tricked
• Orchid flowers resemble female wasps males
  attempt copulation the more orchids the wasps
  mate with, the more pollination occurs
• Good example of coevolution

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Animal Pollinators

• The Scottish broom flower has a tripping
  mechanism that arches the stamens over the bee
  and dusts it with pollen, some of which will rub
  off onto the stigma of the next flower the bee
  visits

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Double Pollination

• After pollen grain lands on stigma, the
  generative cell divides by mitosis into two
  haploid sperm cells
• 1 sperm fertilizes egg forms the zygote (2n)
• 1 sperm fertilizes polar nuclei forms endosperm
  (3n)

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Double Pollination

• Double fertilization ensures that the endosperm
  will develop only in ovules where the egg has
  been fertilized.
• This prevents angiosperms from squandering
  nutrients in eggs that lack an embryo

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Seeds

• After double fertilization, the embryo develops
  to a point and then enters a dormancy period
• During this time, the embryo is housed in a
  tough, protective coating seed coat
• It will remain as the seed until germination,
  usually brought about by the absorption of water
• Seeds allow parent plants to disperse offspring
  and wait until environmental conditions are
  favorable for growth

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Seeds

• In bean seeds (dicot), the embryo consists of an
  long structure, the embryonic axis, attached to
  cotyledons
• Below the point at which the cotyledons are
  attached, the embryonic axis is called the
  hypocotyl above it is the epicotyl
• Tip of the epicotyl is the plumuleshoot tip with
  a pair of mini leaves
• End of the hypocotyl is the
  radicle, or embryonic root

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Seeds

• Monocots have a single cotyledon called a
  scutellum
• Embryo of a grass seed is enclosed by two
  sheaths, a coleorhiza, which covers the young
  root, and a coleoptile, which cover the young
  shoot

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Fruits

• Develop due to hormonal changes after
  fertilization
• Usually develop only after fertilization
• Designed to protect the seeds and aid in seed
  dispersal by wind or animals

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Fruits

• Fruits are simply any structure related to or
  resulting from the ovary of a flower (Yes! That
  includes many of the common vegetables)

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Seed Dispersal

• Fruits aid in seed dispersal based on how the
  fruits develop
• Lightweight fruits allow wind dispersal
• Dandelions and Maples

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Seed Dispersal

• Floating fruits allow water dispersal
• Coconuts

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Seed Dispersal

• Clingy fruits allow animal dispersal
• Fruits grab the animal (cockleburs, jumping
  cholla)

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Seed Dispersal

• Tasty fruits allow animal dispersal
• Fruits entice the animal to eat it (mistletoe and
  birds)

• Animals eat the fruit and deposit the seeds (in a
  nice pile of fertilizer) in new places
• Why are unripe fruits bitter?

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Seed Dispersal

• Explosive seed pods allow dispersal by the plant
  itself
• Impatients get their name from their behavior