angiosperms and reproduction

REPRODUCTION IN

ANGIOSPERMSTopic 9.3•Flower structure•Pollination•Fertilization•Germination

Flower structure• Flowers are reproductive structures• They have evolved to send and receive pollen

from one flower to another• This is the process of pollination• Flowers are developed from a series of modified

leaves• These leaves are arranged in a rings (whorls)

Types of pollination• Wind• Animal• Water

Bumble bee Bombus hortorum on red clover Trifolium pratense

Yorkshire fog grass Holcus lanatus

Animal pollination• Usually insects• Also other flying

animals • e.g. hummingbirds or

fruit bats

Cerambycid beetle pollinating bramble Rubus fruticosus

Flower structureStigma

Style

OvaryPetal

Sepal

Filament

Anther

Pollination• Pollen grains contain the

male gametes of the plant• They are picked up by a

pollinator and transferred to another flower

• Plants tend to specialise in pollinators

• This ensures the pollen is delivered to same species of plant

Yellow archangel Lamiastrum galobdolon being pollinated by a bumble bee Bombus hortorum

Pollination

• Most species of flowering plants are hermaphroditic• Pollen from a flower could land on the stigma of the

same flower or another flower on the same plant = self-pollination• Less genetic variation in species

• Pollen transferred from the anther on one flower to the stigma of another flower on a different plant = cross-pollination

The honey bee Apis melifera on marsh thistle Cirsium palustris

Fertilization• Pollination ≠ Fertilization• The male gamete (the male nucleus) has to get

to the egg cell• The egg cell lies in an ovule in an ovary at the

centre of the plant• The pollen grain germinates on the stigma• It grows a pollen tube down the style• It male nuclei travel down the pollen tube to the

ovule

Fertilization

Style

StigmaPollen grain

Ovule

Embryo sac

Pollen tubeOvary

Fertilization

Egg cell

Polar nuclei

Embryo sac

Micropyle

Fertilization

Pollen grains of the daisy Bellis perennis

Fruits and seed dispersal

Animal dispersal Strawberry Fragaria vesca

Wind dispersal Ragwort Senecio

Explosive dispersal Bird’s foot trefoil Lotus corniculatus

Animal dispersalWood avens Geum urbanum

Conditions for seed germination

Other seeds require more specific conditions:• Fire • Freezing • Passing through digestive system of a seed

dispersing animal • Washing to remove inhibitors (beans) • Erosion of the seed coat (Poppy)

Seeds require a combination of certain conditions to germinate: • Oxygen for aerobic respiration• Water to metabolically activate the cells• Temperature for optimal function of enzymes• Each seed has its own particular combination of the above three

factors

• Water is absorbed (imbibition)• Giberillin (plant growth hormone) is

made• Giberillin causes enzymes to be made

(amylase)• Starch is hydrolysed to maltose which

can then be absorbed by the young plant• Maltose can be further hydrolysed to

glucose for cellular respiration or polymerised to form cellulose for the cell walls.