4.2 - Sexual Reproduction in Plants

Flower

Organ responsible for sexual reproduction in a plant

Female gamete

Ovule

Male gamete

Pollen

Structure of flowers

Insect- VS. Wind-pollinated flower

Location of development of pollen

Anther

Location of development of an ovule

Ovary

Process of development of a pollen grain

• Pollen mother cells develop by mitosis.

• Meiosis occurs to produce a tetrad of 4 haploid cells.

• In each haploid, pollen grain mitotic division of the nucleus forms a generative nucleus (that mitotically divides to produce 2 male nuclei) and a tube nucleus.

• Tension in lateral grooves increases as the anther dries out. Dehiscence occurs when walls of the pollen sac curl away exposing pollen grains to wind or insects.

Tapetum

A layer of cells that provide nutrients to developing pollen grains

Draw a labelled diagram of an anther

Draw a flow diagram of the development of pollen

Process of development of an ovule

• Megaspore mother cell undergoes meiosis

• Forms 4 haploid nuclei

• 3 degenerate

• 1 undergoes 3 mitotic divisions

• Forms 8 nuclei that are genetically identical;

  • Cell membranes form around six of the nuclei to form;

  • One of these cells becomes the ovum,

  • two synergids

  • 3 antipodal cells

  • remaining two remain as free polar nuclei.

Draw a labelled diagram of an ovary

Pollination

Transfer of pollen from an anther to a stigma

Self-pollination VS Cross-pollination

Ways in which plants prevent self-pollination/promote cross-pollination

• Chemical self-incompatibility (gametes from the same plant cannot combine. gametes from the same parent plant are unable to fuse and form a zygote or, if the zygote forms, then it fails to develop.)

• Irregular flower structure

Explain the process of fertilisation

• Germination of a pollen grain on a compatible stigma

• Pollen grain on a compatible stigma produces hydrolase enzymes, forming a pollen tube leading to the micropyle of the embryo sac

• Entry of the pollen tube into the embryo sace through the micropyle

• One male gamete enters the embryo sac + fuses with the female gamete to produce a diploid zygote

• The second male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus

Draw a diagram of the process of fertilisation

Formation of a seed

• Ovule developing into a seed

• Diploid zygote divides by mitosis to form the diploid embryo, insuring of a plumule, radicle and one or two cotyledons

• The triploid endosperm nucleus divided by mitosis to form endosperm tissue

• Interment develop into the testa

• Micropyle remains as a pore in the testa

• Ovary wall develops into a fruit wall enclosing the seeds

Development of the ovule after fertilisation

Seed

Development of diploid zygote after fertilisation

Divides by mitosis to form the diploid embryo. This is made of the plumule, radical and 1 or 2 cotyledons

Development of triploid endosperm nucleus after fertilisation

Divides by mitosis to form endosperm tissue, an important food storage tissue

Development of integuments after fertilisation

Testa

Development of micropyle after fertilisation

Pore in the testa

Development of ovary after fertilisation

Develops into a fruit wall, enclosing seeds

Draw a fully labelled diagram of a broad bean

Draw a fully labelled diagram of a maize seed

Benefit of different mechanisms of dispersal of seeds

Reduces competition following germination and increases the chance of growth into mature plants

Germination definition

the series of biochemical and physiological processes through which a seed becomes a photosynthesising plant, independent of the food stores in the cotyledons

Conditions for germination

• Seeds remain dormant until suitable conditions are present

• Temperature; a suitable temperature would be between 5 and 30•C. Optimum temperature for the enzymes needed in germination

• Water; required to make cells turgid to transport substances and to mobilise enzymes

• Oxygen; required for aerobic respiration

Temperature for germination

A suitable temperature would be between 5 and 30•C. Optimum temperature for the enzymes needed in germination

Water needed for germination

Required to make cells turgid

Oxygen needed for germination

Required for aerobic respiration

Process of germination in a non-endospermic seed

Broad bean

• Water imbibed/enters through the micropyle

• Cotelydons swell

• Test splits to allow entry of more oxygen for aerobic respiration

• Food reserves from cotyledons, starch and proteins, are mobilised through hydrolysis (and lipids in some seeds)

• Starch and protein used as sources of energy for use in respiration and growth of plumule and radical

Process of germination an endosperm’s seed

• Water imbibes/enters the seed

• Gibberellin is released by the embryo

• Diffuses to the aleurone layer which contains proteins

• Amylases produced to break down stored starch in endosperm

• Glucose and other nutrients diffuse to the embryo where they are used in aerobic respiration and growth

Gibberellin

Plant hormone involved in the process of germination

Hormone used in germination of an endospermic seed

Gibberellin

Male reproductive organ in plants

Anther

Female reproductive organ in plants

Ovary

Sepal

Each of the parts of the calyx of a flower, enclosing the petals

Corolla

Composed of petals

Function of the Corolla

Colourful and scented to attract insects

Corolla in a wind pollinated flower

Petals are often lacing or if present small and green

Carpel: male or female?

Female

Stamen: male or female?

Male

Carpel

Made of a sticky stigma to collect pollen from the insects bodies

Carpel of a wind pollinated flower

Large and feathery

Hangs outside the flower to collect pollen blown past

Receptacle and calyx

Outer ring of sepals that cover the flower in bud

Stamen

Made of the anther and filament is tucked inside the flower so the insect rubs past picking up pollen

Anther in a wind pollinated flower

Large and hangs outside the flower, so the small light pollen gets carried away

Draw a fully labelled diagram of the germination of an endospermic seed

Dormancy and germination

Water content of seeds is very low and is the major factor that prevents germination. Seeds will remain dormant until suitable conditions are present

Example of non-endospermic seeds

Broad bean

Example of endospermic seeds

Maize