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9.4 Reproduction in Plants Cheat Sheet by

IB 2016 syllabus plant biology

Plant reprod­uction

Success in plant reprod­uction depends on pollin­ation, fertil­ization and seed dispersal.
Plants can reproduce in a variety of different ways:
- Vegetative propagation
- Spore formations
- Pollen transfer
Sexual reprod­uction in flowering plants involves the transfer of pollen to an ova, and involves three distinct phases.
The transfer of pollen grains from an anther to a stigma.
Many plants possess both male and female structures (monoecious)and can potent­ially self-pollinate.
from an evolut­ionary perspe­ctive, cross-­pol­lin­ation is preferable as it improves genetic diversity.
Fusion of a male gamete nuclei with a female gamete nuclei to form a zygote.
In plants, the male gamete is stored in the pollen grain and the female gamete is found in the ovule.
Seed dispersal
Fertil­i.s­ation of gametes results in the formation of a seed, which moves away from the parental plant.
Seed dispersal reduces the compet­ition for resources between germin­ating seeds and the parental plant.
There are a variety of seed dispersal mechan­isms, including wind, water, fruits and animals.
Seed structure varies depending on the mechanisms of dispersal employed by the plant.
Most flowering plants use mutual­istic relati­onships with pollin­ators in sexual reprod­uction.
Cross-­pol­lin­ation involves transf­erring pollen grains from one plant to the ovule of a different plant.
Pollin­ators are animals that are involved in a mutual­istic relati­onship with the flowering plant where both species benefit from the interaction.
The plant gains a means of sexual reprod­uction whit the animal gains a source of nutrition.
Common pollin­ators include birds, bats and insects.
Flowers may be structured to optimise access for certain pollin­ators.


The switch to flowering is a response to the length of light and dark periods in many plants.
Phytoc­hromes are leaf pigments which are used by the plant to detect periods of light and darkness.
Photop­eri­odism is the response of the plant to the relative lengths of light and darkness.
Phytoc­hromes exist in two forms - an active form and an inactive form, with the active form being Pfr which is formed from the inactive form, Pr at 660nm. Pfr breaks down into Pr at ~725nm, but will also gradually revert to the inactive form in the absence of light.
Because sunlight contains more red light than moonlight, the active form is predom­inant during the day.
Only Pfr can cause flowering, however its action differs in certain types of plants.
Plants can be classed as short-day or long-day plants based on the night length required for flowering.
Short-day plants flower when the days are short – hence require the night period to exceed a critical length, meaning that Pfr inhibits flowering.
Long-day plants flower when the days are long – hence require the night period to be less than a critical length, meaning Pfr promotes flowering.
Methods used to induce short-day plants to flower out of season
Hortic­ult­ura­lists can manipulate the flowering of short-day and long-day plants by contro­lling the exposure of light.
Long-day plants require periods of darkness to be less than an uninte­rrupted critical length.
These plants will tradit­ionally not flower during the winter and autumn months when night lengths are long.
Hortic­ult­ura­lists can trigger flowering in these plants by exposing the plant to a light source during the night.
Short-day plants require periods of darkness to be greater than an uninte­rrupted critical length.
These plants will tradit­ionally not flower during the summer months when night lengths are short.
Hortic­ult­ura­lists can trigger flowering in these plants by covering the plant with an opaque black cloth for ~12 hours a day.


Flowering involves a change in gene expression in the shoot apex.
Flowers are the reprod­uctive organs of angios­per­mop­hytes and develop from the shoot apex.
Changes in gene expression trigger the enlarg­ement of the shoot apical meristem.
This tissue then differ­ent­iates to form the different flower structures - spears, petals, stamen and pistil.
The activation of genes respon­sible for flowering is influences by abiotic factors - typically linked to the seasons.
Flowering plants will typically bloom when a suitable pollinator is most abundant.
The most common trigger for a change in gene expression is day/night length (photo­per­iod­ism).

Flower structure

Drawing of half-views of animal­-po­lli­nated flowers.
Monoecious flowers contain both male and female reprod­uctive struct­ures.
Dioecious flowers only possess one structure.
Stamen is the male part of the flower.
Anthers are the pollen producing organs of the flower, with the pollen being the male gamete.
Filaments are the slender stalks that support the anther and make it accessible to pollin­ators.
Pistil is the female part of the flower.
Stigma is the sticky receptive tip of the pistil that is respon­sible for catching the pollen.
Style is the tube-s­haped connection between the stigma and the ovule that elevates the stigma to help catch pollen.
Ovule is the structure that contains the female gametes, and will develop into the seed after fertil­isa­tion.
Support structures in flowers
Petals are brightly coloured modified leaves, which function to attract pollinators.
Sepal is the outer covering that protects the flower when in the bud.
Peduncle is the stalk of the flower.

Structure of a flower

Seed structure

an outer seed coat that protects the embryonic plant
a small pore in the outer covering of the seed, that allows for the passage of water
contains the food stores for the seed and forms the embryonic leaves
the embryonic shoot (also called the epicotyl)
the embryonic root

Internal structure of a seed


Germin­ation is the process by which a seed emerges from a period of dormancy and begins to sprout.
Essential conditions
for aerobic respir­ation
to metabo­lically activate the seed
seeds require certain temper­ature conditions to sprout
seeds require a suitable soil pH to sprout
Specia­lised conditions
some seeds will only sprout after exposure to intense heat
some seeds will only sprout after periods of intense cold
some seeds require prior animal digestion to erode the seed coat before the seed will sprout
some seeds may be covered with inhibitors and will only sprout after being washed to remove the inhibitors
seeds are more likely to germinate if the seed coat is weakened from physical damage


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