4.2: Plant reproduction

Angiosperms

Term for flowering plants. Flowers are their chosen reproductive structure.

Sepal

Modified leaves which protect the flower when it’s in a bud. They are normally green, but can be coloured.

Calyx

Outermost ring of structures, made up of sepals. Normally green, but can be coloured.

Petals

Modified leaves which are colourful to attract pollinators. They can range in colours and size.

Corolla

Multiple petals, second outermost ring of structures. Range in size and colour. Nectaries, if present, will be at the base of this.

Nectaries

At the base of the corolla. Release scented nectar to attract pollinators. This nectar is mainly made up of sucrose.

Stamen

Name for the male parts of a flower. Includes the filament and anther, and produces pollen grains.

Filament

Supports the anther. Contains vascular tissue, which transports sucrose, mineral ions and water to developing pollen grains. Part of the stamen.

Anther

Produces pollen grains. Contains 4 pollen sacs arranged in pairs, side by side. When mature, undergoes dehiscence to release pollen. Part of the stamen.

Carpel

The female parts of a flower. Includes the ovary, style and stigma, and produces ovules. A closed structure.

Ovary

Part of the carpel which surrounds the ovules, and is connected by the funicle. After fertilisation, this becomes the fruit, and can become sweet and pigmented, or dry and hard. The testa will fuse with this in monocotyledons.

Style

A long structure, which ends in the receptive surface, the stigma. Part of the carpel.

Stigma

The receptive surface for pollen grains. Part of the carpel.

Pollinators

Type of pollination which relies on attracting insects, such as bees, via large, colourful flowers, scent and nectar. When insects drink the nectar, they rub on the anther and sticky, sculptured pollen grains attach. When the bee then rubs against a ripe stigma, cross-pollination has occurred.

Wind

Type of pollination which relies on wind to move their small and light pollen. Stigmas are feathery and hang outside the flowers to catch pollen grains. They normally group together in an inflorescence.

Inflorescence

A cluster of flowers on a branch. Normally occurs in wind-pollinated flowers.

Self

Type of pollination that results in inbreeding. Lowers genetic diversity and variation, increases chances of harmful genes spreading, requires less energy and maintains successful genomes.

Cross

Type of fertilisations which results in outbreeding, and occurs in most Angiosperms. Increases genetic variation and likelihood of adaptation, reduces chances of the spread of harmful genes but requires more energy.

Dichogamy

When the stigma and stamen ripen at different times to avoid self-pollination. Two types, protandry and protogyny.

Protandry

Type of dichogamy where stamens ripen first. Occurs in daisies.

Protogyny

Type of dichogamy where stigmas ripen first. Is rarer. Occurs in bluebelles.

Receptacle

Helps support the flower.

Epidermis

Outermost layer of the anther.

Pollen sacs

Each anther contains four - where the pollen grains are formed. Has two layers, the fibrous layer and the tapetum, which provides nutrients.

Fibrous

Outermost layer of the pollen sacs.

Tapetum

Innermost layer of the pollen sacs. Provides nutrients to the cell, and helps form the pollen cell wall. This is chemically resistant, tough, resists desiccation to prevent it from drying out before reaching another flower and blocks UV light, which is helpful as pollen grains move at high altitudes.

Dehiscence

How pollen grains leave anthers. Outer layers of the anthers dry out, causing tension in the lateral grooves. This pulls the walls apart, and the edges of pollen sacs curl away. This forms an opening known as the stomium which exposes pollen grains.

Lateral grooves

Where dehiscence occurs. Become stomium which exposes pollen grains to pollinators or the wind.

Stomium

Openings caused by dehiscence. Expose pollen grains to pollinators or the wind.

Pollen mother

Beginning cell of male gametogenesis. Undergoes mitosis to create the tetrad of haploid pollen cells.

Tetrad

Amount of haploid pollen cells created by meiosis of the pollen mother cell. Undergo mitosis to produce two nuclei; the generative and tube nucleus.

Exine

The outermost layer of the pollen grain cell wall, which is tough and waterproof. Has gaps known as pits where gas exchange occurs.

Intine

Innermost layer of the pollen grain cell wall. A thinner layer.

Ovule

Contains the female gamete, and is in the ovary. Made up of the two integuments, the micropyle, nucellus and the embryo sac. After fertilisation, becomes the seed.

Integuments

Part of the ovule. The two cell layers which surround the embryo sac. Have a gap known as the micropyle. After fertilisation, they join with lignin and dry out to make a hard and waterproof layer known as the testa.

Micropyle

Part of the ovule. Gap in the two integument layers where the pollen tube enters the embryo sac. Remains after the ovule becomes a seed.

Embryo sac

Part of the ovule. Contains the eight nuclei and is where the female gamete is developed.

Nucellus

Part of the ovule. Surrounds the embryo sac and provides nutrients. Is replaced by the endosperm tissue after double fertilisation.

Megaspore mother

Beginning cell of female gametogenesis. Undergoes meiosis to make four haploid cells known as megaspores, 3 of which degenerate.

Megaspores

Created by the meiosis of the megaspore mother cells. 3 of these disintegrate. The one remaining undergoes mitosis three times to create eight haploid nuclei.

Polar

Nuclei that make up two of the eight made. They fuse into one diploid cell. They are in the centre of the embryo sac.

Synergids

Nuclei that make up two of the eight made. They are located at the bottom of the embryo sac, opposite the antipodals and next to the oosphere.

Oosphere

One of the eight nuclei made - the female gamete. Positioned above and between the synergids in the embryo sac.

Antipodals

Three of the eight nuclei made. Positioned at the top of the embryo sac, the opposite side of the synergids.

Pollen grain

Contains the male gamete. Made up of the exine, intine, pits and the generative and tube nuclei. Lands on the stigma, and germinates in the sucrose before creating a pollen tube. Has two nuclei.

Pollen tube

Formed by the tube nucleus in pollen grains through a pit, and moves up a chemoattractant gradient by releasing hydrolases which digest through the stigma and the style to the micropyle. It then opens and releases the two generative nuclei into the embryo sac.

Tube

Nucleus in the pollen grain that releases hydrolases, such as proteases and cellulases, to digest the stigma and style. Uses the digestion products for the pollen tube. Disintegrates once it reaches the embryo sac.

GABA

A chemoattractant released by the ovule to attract the pollen tube to the micropyle for double fertilisation.

Generative

Nucleus in the pollen grain that undergoes mitosis. One of these fertilises the oosphere to form the zygote, and one fertilises the polar nucleus to form the endosperm nucleus, which is triploid.

Endosperm

Formed by the fusion of generative nucleus and the polar nucleus, which is triploid. Multiplies rapidly to form the ——— tissue, which takes over from the nucellus in providing nutrition for the embryo. In monocotyledons, it remains the food store. In dicotyledons, it is absorbed by the two seed leaves. Nutrients last until the embryo can photosynthesise on its own.

Plumule

Part of the embryo - the developing shoot. Is negatively geotropic and positively phototropic, meaning it grows upwards. Has a hook, which straightens once above ground to unfurl the leaves.

Radicle

Part of the embryo - the developing root. Is positively geotropic and negatively phototropic, meaning it grows downwards.

Cotyledon

Seed leaves. Plants can be split into two categories, mono—— and di—— based on how many of these they have. Nutrients last until the embryo can photosynthesise on its own.

Testa

Formed after fertilisation, when integuments dry, harden, and become waterproof with lignin deposits. Monocotyledons fuse to the ovary wall via this. It is chemically resistant and protects the embryo. Ruptures when embryo cells become turgid, allowing the plumule and radicle leave.

Dicotyledons

Plants which have two seed leaves, with the embryo lying between them. AKA non-endospermic seeds, as the cotyledons absorb the endosperm. For example, broad beans.

Monocotyledons

Plants which have one seed leaf, which remains small and does not develop further. They become dormant, dropping water content to below 10% to reduce metabolic rate. They will not germinate until they are in suitable conditions. For examples, maize and other cereal grains.

Hilum

A scar or mark left on the seed from where the funicle attached it to the fruit / ovary.

Funicle

Attaches the ovule and the later developed seed to the ovary wall. Leaves a scar or mark known as the hilum.

Wind

Method of dispersal where seeds are designed to have pores, sails or parachute-like structures to be blown by the wind. An example is dandelions.

Transport

Method of dispersal where animals eat seeds and disperse them via their faeces. Fruits evolve to be tasty and brightly coloured to attract this. Some species can only germinate after scarification has occurred. An example is cherries.

Scarification

A process where the testa is digested by acid and enzymes. Some seeds can only germinate after this occurs.

Rolling

Method of dispersal where the fruit breaks open to release the seed, which falls and moves away from the parent plant. An example is conkers.

Bursting

Method of dispersal where legume pods dry out, split and the seeds scatter. Some pods rotate as they burst open. An example is pea pods.

Water

Method of dispersal where seeds are buoyant via air cavities, and float in the water when they fall. An example is coconut trees.

Carrying

Method of dispersal where hooked seeds attach to animal coats to be carried away. An example is burdock.

Vernalisation

Process of inhibitors being broken down in very cold weather, to be germinated in the spring. Inhibitors can be in seeds or fruit.

Vacuolate

The formation of vacuoles. Done during germination in order for cells to become turgid.

Imbibed

Term that means absorbed via a pore. During germination, this is done with water via the micropyle in order to allow for vacuolation and enzyme mobilisation via the xylem and phloem.

Amylose and protease

Enzymes produced for germination to occur, breaking down proteins in the aleurone layer into amino acids for protein synthesis, and starch in the endosperm into maltose and glucose to be respired. These products are transported to the apical meristems of the radicle and plumule and the embryo.

Gibberelic acid

Secreted by the barley embryo to activate genes, and therefore transcription and translation, in the outer aleurone layer. This triggers protease to break it down into amino acids, which then produces amylase, which breaks down sugars in the endosperm tissue. The seed therefore ends dormancy.

Apical meristems

Where amino acids and maltose are transported in the plumule and radicle - the uppermost unspecialised tissue.

Hook

The shape the plumule grows out of the soil to prevent soil abrasion to the leaves. Straightens once above ground, allowing leaves to unfurl and photosynthesis to begin.

Pollination

The transfer of pollen grains from the anther to the mature stigma of a plant of the same species. Can occur via pollinators or the wind.

Aleurone

Outer layer of a monocotyledon seed, where gibberellic acid diffuses in order to switch on genes and begin transcription and translation of enzymes to begin germination.