Chapter 38
Plant Reproduction and Development
Reproduction and Development
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
Reproduction and Development
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
Reproduction and Development
Reproduction and Development
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
Reproduction and Development
Flower Structure
Reproduction and Development
Flower Anatomy Sepals and petals are nonreproductive
organs
– Sepals – protect the other three, the floral bud
– Petals – attract pollinators and act as “landing pads”
Reproduction and Development
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)
Reproduction and Development
Flower Anatomy Complete flowers – have all four floral
organs
– Ex: Trillium
Incomplete flowers – missing one or more of the four floral organs
Reproduction and Development
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)
Reproduction and Development
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
Reproduction and Development
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
Reproduction and Development
Gamete Formation Development
of angiosperm gametophytes involves meiosisand mitosis
Reproduction and Development
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
Reproduction and Development
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
Reproduction and Development
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
Reproduction and Development
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
Reproduction and Development
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
Reproduction and Development
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
Reproduction and Development
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
Reproduction and Development
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)
Reproduction and Development
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
Reproduction and Development
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
Reproduction and Development
Reproduction and Development
Reproduction and Development
Reproduction and Development
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
Reproduction and Development
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)
Reproduction and Development
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
Reproduction and Development
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
Reproduction and Development
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 plumule:shoot tip with a pair of mini leaves– End of the
hypocotyl is the radicle, or embryonic root
Reproduction and Development
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
Reproduction and Development
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
Reproduction and Development
Fruits Fruits are simply any structure related to or
resulting from the ovary of a flower (Yes! That includes many of the common “vegetables”)
Reproduction and Development
Seed Dispersal Fruits aid in seed dispersal based on how
the fruits develop
– Lightweight fruits allow wind dispersalDandelions and Maples
Reproduction and Development
Seed Dispersal– Floating fruits allow water dispersal
Coconuts
Reproduction and Development
Seed Dispersal–Clingy fruits allow animal dispersal
Fruits “grab” the animal (cockleburs, “jumping” cholla)
Reproduction and Development
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?
Reproduction and Development
Seed Dispersal–Explosive seed pods allow dispersal by
the plant itselfImpatients – get their name from their behavior