Chapter 38
Plant 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
meiosis
and mitosis
Reproduction and Development
Gamete Formation
The male gametophytes are spermproducing structures called pollen grains,
which form within the pollen sacs of
anthers
 The female gametophytes are eggproducing 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 dispersal
Dandelions 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 itself
Impatients – get their name from
their behavior
Reproduction and Development