CHAPTER 34

PLANT REPRODUCTION AND

GROWTH

ANGIOSPERM REPRODUCTION

• Reproduction in flowering plants, the angiosperms, can be asexual or sexual.• Asexual reproduction is common in stable

environments.• This vegetative reproduction results when

new individuals are simply cloned from parts of the parent.

• Asexual reproduction allows individuals to reproduce with a lower investment of energy than sexual reproduction.

ANGIOSPERM REPRODUCTION

• There are many forms of vegetative reproduction: • Runners are slender stems that grow along the

soil surface.• Rhizomes are underground horizontal stems

that create a network, giving rise to new shoots.• Suckers are produced by roots and give rise to

new plants.• Adventitious plantlets arise from

meristematic tissue located in the notches of leaves.

VEGETATIVE REPRODUCTION

RunnerRhizome

ANGIOSPERM REPRODUCTION

• Sexual reproduction in plants involves an alternation of generations.• The diploid sporophyte generation gives rise to

a haploid gametophyte generation, which is enclosed within the tissues of the sporophyte.

Parentsporophyte tissue(main plant bodyand outer whorlsof flowers)

Femalegametophyte(embryo sac)is inside baseof flower

Malegametophytes(pollen grains)are on thesefilaments

ANGIOSPERM REPRODUCTION

STRUCTURE OF THE FLOWER

• Most flowers contain male and female parts.• The male parts are called stamens.• The female part is called the carpel.

• Flowers that contain only male or only female parts are known as imperfect.

THE STRUCTURE OF A FLOWER

Stigma

Style

Ovary

Anther

Stamen

(a)

Carpel

(c)

(b)

b: © David Sieren/Visuals Unlimited; c: © Barbara Gerlach/Visuals Unlimited

STRUCTURE OF THE FLOWER

• Pollen formation occurs in the anthers.• Inside the anthers are pollen sacs, which

contain microspore mother cells.• Each microspore mother cell undergoes meiosis

to form four haploid microspores.• These microspores then undergo mitosis to

form pollen grains that contain a generative cell and a tube cell nucleus.• The generative cell will later divide to form

two sperm cells.

STRUCTURE OF THE FLOWER

• Egg formation occurs in the ovary, which contains the ovule.– Each ovule contains a megaspore

mother cell that undergoes meiosis.• Only one megaspore survives to undergo

repeated mitotic divisions that produce eight haploid nuclei.

– These nuclei are enclosed in an embryo sac, where the nuclei are precisely arranged.

Formation of pollen and egg

1

2 3

12

34

Pollen grains (n)

Tube cellnucleus

Mitosis

Microspores (n)Microsporemother cell (2n)

Generativecell

Meiosis

Pollen sacs

Anther

Stamen

Carpel

Ovary

Antipodals

Eggcell

8-nucleate embryo sac (n)Degeneratedmegaspores

Survivingmegaspore

Megaspores (n)Megasporemother cell (2n)

Ovule

Meiosis

Polarnuclei

Mitosis

Synergids

2 3

pollen tube

Tube cell

Sperm cells

Tube cell nucleusPollen tube

Growth of

Pollination

Pollen grain

Stigma

Style

Ovary

Carpel

Embryosac

1

Tubecell

Ovule

Generativecell

4 5

Antipodals

Egg cell

Synergids

Polar nuclei

Release of sperm cells Double fertilization

Zygote(2n)

Endosperm(3n)

GAMETES COMBINE WITHIN THE FLOWER

• Pollination is the process by which pollen is transferred from the anther to the stigma.

FERTILIZATION

• Pollen grains adhere to the sticky surface of the stigma and begin to grow a pollen tube.

• The pollen tube pierces the style and grows until it reaches the ovule in the ovary.

• When the pollen tube reaches the entry to the embryo sac, it releases two sperm cells.• One sperm fertilizes the egg while the other sperm

fuses with the polar nuclei to form endosperm.• This process of using two sperm cells in fertilization

is called double fertilization.

GAMETES COMBINE WITHIN THE FLOWER

• Many angiosperms use animals to carry pollen grains from flower to flower.• These pollinators may

be rewarded for their efforts with food (e.g., nectar).

• Coevolution has occurred between plants and pollinators.

GAMETES COMBINE WITHIN THE FLOWER

• In some angiosperms and in all gymnosperms, pollen is dispersed by wind and reaches the stigmas passively.• The individuals of a given

plant species must grow where there is ample wind and grow relatively close together.

SEEDS

• Development is the entire series of events that occurs between fertilization and maturity.• The first stage of development is active cell

division to form an organized mass of cells, the embryo.

2 3 4

567

8 Root apicalmeristem

Root apex (radicle)

Endosperm

Basal cell

Suspensor

Endosperm (3n)

Embryo

Cotyledons

Protoderm

Cotyledon

Shoot apicalmeristem

Procambium

Groundmeristem

Hypocotyl

Cotyledons

Triploid endospermmother cell

First celldivision

Pollentube

Zygote(2n)

1Polarnuclei

Egg(n)

Micropyle

Sperm (n)

DEVELOPMENT IN AN ANGIOSPERM EMBRYO

SEEDS

• The integuments that form the outermost covering of the ovule develop into a seed coat.• This layer is relatively impermeable and

encloses the dormant embryo within the seed, together with a source of food.

• Germination cannot take place until water and oxygen reach the embryo.• This assures that the seed will germinate when

conditions are favorable for a plant’s survival.

FRUIT

• During seed formation, the flower ovary begins to develop into fruit.• Fruits form in many ways and exhibit a wide

array of modes of specialization.• Fleshy fruits are normally dispersed by birds

and other vertebrates.• Some fruits are dispersed by wind or by

attaching themselves to the fur of mammals or the feathers of birds.

• Some fruits are dispersed by water.

TYPES OF FRUITS AND COMMON MODES OF DISPERSION

GERMINATION

• When a seed encounters conditions suitable for its germination:• It first absorbs water.• Once the seed coat ruptures, aerobic

respiration begins.• The roots emerge first.• Cotyledons emerge, in dicots, from

underground along with the stem.

DEVELOPMENT OF ANGIOSPERMS

DICOT

First leaf

Cotyledon

Hypocotyl

Seedcoat

Primaryroots

Secondaryroots Primary

root

Radicle

CotyledonColeoptile

Witheredcotyledons

MONOCOT

Adventitiousroot

Firstleaf

GROWTH AND NUTRITION

• Plants require a number of nutrients.• Macronutrients are needed in large amounts.• Micronutrients are needed in trace amounts.• These nutrients are called essential because

the plant cannot manufacture them.

PLANT HORMONES

• Plant hormones control the expression of some plant genes.• Hormones in plants are produced in tissues that

are not specialized for that purpose and carry out many other functions.

PLANT HORMONES

• At least five major kinds of hormones are found in plants

• auxin• gibberellins• cytokinins• ethylene• abscisic acid

AUXIN

• Phototropism is the growth of plants toward light.• Charles Darwin and his son Francis performed

experiments that suggested that a substance caused the plant to bend if exposed to light.

• The substance was later identified to be auxin.

THE DARWINS’ EXPERIMENT WITH PHOTOTROPISM

4

1 2

3

LightLight

Charles and Francis Darwin found that a young grassseedling normally bent towards the light.

If the tip of the seedling was covered with a light proofcap, the seedling did not bend toward the light.

Light Light

When the tip of the seedling was covered with atransparent cap, the bending did occur.

When the Darwins placed a lightproof collar below thetip, the seedling bent toward the light.

AUXIN

• Frits Went worked out how auxin controls plant growth.• Auxin causes the tissues

on the side of the seedling into which it flows to grow more than those on the opposite side.

• The side of the plant in the shade has more auxin and divides more, causing the plant to bend towards the light.

Shaded sideof seedlingLight

Lighted sideof seedling

PHOTOPERIODISM AND DORMANCY

• Photoperiodism is a mechanism by which organisms measure seasonal changes in relative day and night length.• Plants’ flowering responses fall into three basic

categories in relation to day length:• Long-day plants flower when days become

longer in the summer.• Short-day plants flower when days become

shorter in the fall.• Day-neutral plants produce flowers without

regard to day length.

PHOTOPERIODISM AND DORMANCY

• Plants have the ability to stop growing altogether when conditions are not favorable.• This is called dormancy.• In temperate zones, dormancy is generally

associated with winter when low temperature and the freezing of water make it impossible for plant growth.

TROPISMS

• Tropisms are directional and irreversible growth responses to external stimuli• Gravitropism causes

stems to grow upward and roots to grow downward.

• Thigmotropism is the response of plants to touch.