Pollen cannot swim, walk or fly. So a flower has to hire a courier or gamble on the weather — and the two strategies produce flowers that look nothing alike. Once you see why each feature exists, you never have to memorise the tables.
📚 What you need to know
Flowers are the reproductive organ of the plant and usually contain both male and female parts.
Pollen contains the male gamete nucleus, but unlike sperm it is not capable of locomotion.
Pollination is the transfer of pollen from the anther to the stigma. The two main mechanisms are transfer by insects (or other animals) and transfer by wind.
Insect-pollinated flowers have large bright petals, scent and nectar, and sticky or spiky pollen.
Wind-pollinated flowers have small dull petals, no scent or nectar, huge amounts of smooth light pollen, and anthers and stigmas outside the flower.
Cross-pollination transfers pollen to another plant of the same species and improves genetic variation.
Self-pollination gives less variation, so plants use different maturation times, separate male and female flowers and self-incompatibility mechanisms to avoid it.
Why flowers look the way they do
Every feature of a flower is an answer to one question: how does the pollen get from here to a stigma? Insect-pollinated flowers pay a courier. They produce nectar, a sugary substance made at the base of the petals that provides insects with energy, and they advertise it with colour and scent.
An insect pushing into a flower for nectar brushes against the anthers, which deposit sticky pollen onto its body. When it visits the next flower it brushes against that flower’s stigma and deposits some of the pollen it is carrying. Pollination is, from the plant’s point of view, an accident that it has engineered.
Wind-pollinated flowers do not pay anyone. When ripe, the anthers open and shed their pollen into the open air, and the pollen is blown or carried on air currents until, by chance, it lands on the stigma of a plant of the same species. The process is far more random, and every feature of the flower compensates for that.
The wind-pollinated anthers dangle in the open air on long filaments; the insect-pollinated ones are tucked inside where a visitor has to brush past them.
Insect-pollinated features
Feature
Insect-pollinated
Petals
Large and brightly coloured, to attract insects
Scent and nectar
Present, enticing insects into the flower and past the stamens to reach the nectar
Number of pollen grains
Moderate, because insects transfer pollen efficiently with a good chance of success
Pollen grains
Larger, sticky and often spiky, so they attach to insects and are carried away
Anthers
Inside the flower, stiff and firmly attached so insects brush against them
Stigma
Inside the flower and sticky, so grains stick to it as an insect brushes past
Wind-pollinated features
Feature
Wind-pollinated
Petals
Small and dull, often green or brown
Scent and nectar
Absent — no need to waste energy producing them when no insects are being attracted
Number of pollen grains
Very large amounts, because most grains never reach another flower
Pollen grains
Smooth, small and light, so they are easily blown by the wind
Anthers
Outside the flower, swinging loosely on long filaments to release pollen easily
Stigma
Outside the flower and feathery, to catch drifting pollen grains
The pattern in one line. Insect-pollinated flowers invest in attraction and precision; wind-pollinated flowers invest in quantity and exposure. Any feature you are asked about can be justified from one of those two.
Cross-pollination and its risk
Cross-pollination is the transfer of pollen from one plant to the stigma of another plant of the same species. Most plants rely on it, because it improves genetic variation.
It carries a risk, though. Cross-pollination depends completely on the presence of pollinators, which is a problem when those pollinators disappear. The decline in bee numbers matters enormously to humans, because bees pollinate a large number of food crops. Wind-pollinated plants do not have this vulnerability.
Preventing self-pollination
Pollen from a flower can land on its own stigma, or on the stigma of another flower on the same plant. That is self-pollination, and it results in less genetic variation than cross-pollination because no new alleles are introduced from other individuals.
Follow that through and you can see why plants avoid it. A lack of variation in the offspring is a disadvantage if environmental conditions change, because it is less likely that any offspring will happen to have adaptations suited to the new conditions.
🧩 Four ways plants avoid self-pollination
Different maturation times — the pollen and the ovules of the same flower ripen at different times, so they cannot fertilise each other.
Separate sexes — some plants produce flowers with only male or only female parts, or the whole plant is one sex.
Distance — in wind-pollinated species the wind carries pollen far from the parent plant.
Self-incompatibility — a genetic mechanism that stops the pollen tube growing when pollen lands on a stigma of the same plant.
How self-incompatibility works
Each plant has a set of genes that controls the growth of a pollen tube. When pollen lands on the stigma of a flower of the same plant, protein interactions occur that prevent the growth of a pollen tube. The block can act at several points:
The pollen grain fails to germinate into a pollen tube at all.
The grain germinates but the tube does not enter the style.
The pollen nuclei reach the ovule but degenerate before fertilisation can occur.
Fertilisation occurs but the embryo degenerates before growth is established.
Worked example
WORKED EXAMPLE
Explain why wind-pollinated flowers produce far more pollen grains than insect-pollinated flowers. [3]
Point 1 — the method is random
Wind-pollinated pollen is released into the air and only lands on a stigma by chance.
Point 2 — the loss
Most grains are therefore never transferred to another flower of the same species.
Point 3 — the compensation
Producing very large amounts of pollen increases the chance that some grains achieve successful pollination. Insects transfer pollen far more efficiently, so fewer grains are needed.
3 marksThe comparison in the last sentence is what turns two marks into three.
WORKED EXAMPLE
Suggest one advantage and one disadvantage to a plant species of a self-incompatibility mechanism. [2]
Advantage
It forces cross-pollination, so offspring receive alleles from two different plants and genetic variation increases, improving the chance that some survive if conditions change.
Disadvantage
An isolated plant with no other member of its species nearby cannot reproduce sexually at all.
2 marksThe disadvantage mirrors the general drawback of sexual reproduction from page one of this topic.
💡 Exam tip
Never just describe a feature — explain why it suits that method of pollination. The reason is the mark.
Sticky and spiky pollen goes with insects; smooth and light goes with wind. Feel the difference physically and you will not forget it.
“Anthers and stigmas outside the flower” is a strong wind-pollination indicator in any unfamiliar diagram.
Link self-pollination to reduced variation, and reduced variation to poor adaptation to change.
Self-incompatibility acts on the pollen tube, not on the pollen landing. Pollen can still arrive.
Bees are the standard example for pollinator decline. Mention food crops if the question is about human impact.
⚠ Common mix-up
Pollination is not fertilisation. It is only the transfer of pollen.
Wind-pollinated flowers are still flowers. They have petals, just small dull ones.
Cross-pollination is not between species. It is between different plants of the same species.
Self-pollination is not a mutation or a fault. It is a normal, if less useful, outcome.
Nectar is not pollen. Nectar is the reward; pollen is the cargo.
Self-incompatibility is genetic, not physical distance between anther and stigma.
Up next: Seed Dispersal & Germination — getting the seed away from the parent, then waking it up again.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
