Chapter 38

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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

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

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

Flower Structure

Flower Anatomy Sepals and petals are nonreproductive

organs

– Sepals – protect the other three, the floral bud

– Petals – attract pollinators and act as “landing pads”

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)

Flower Anatomy Complete flowers – have all four floral

organs

– Ex: Trillium

Incomplete flowers – missing one or more of the four floral organs

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)

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

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

Gamete Formation Development

of angiosperm gametophytes involves meiosisand mitosis

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

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

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

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

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

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

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

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)

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

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

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

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)

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

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

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

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

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

Fruits Fruits are simply any structure related to or

resulting from the ovary of a flower (Yes! That includes many of the common “vegetables”)

Seed Dispersal Fruits aid in seed dispersal based on how

the fruits develop

– Lightweight fruits allow wind dispersalDandelions and Maples

Seed Dispersal– Floating fruits allow water dispersal

Coconuts

Seed Dispersal–Clingy fruits allow animal dispersal

Fruits “grab” the animal (cockleburs, “jumping” cholla)

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?

Seed Dispersal–Explosive seed pods allow dispersal by

the plant itselfImpatients – get their name from their behavior

Chapter 38

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