On this page
Questions on outbreeding devices are short but slippery. Most of them hinge on one distinction: which kind of self-pollination a device prevents. If you know the three kinds of pollination precisely, every device falls into place. This post sets out both, works through four plants, and ends with a short practice routine.
Where is this in the syllabus?
Both the NEET (UG) 2026 syllabus and the CBSE Biology curriculum for 2026-27 list "pollination - types, agencies and examples" and "outbreeding devices" in the chapter on sexual reproduction in flowering plants. The CBSE Class 12 practical list also includes spotting flowers adapted to wind, insect and bird pollination, and controlled pollination by emasculation, tagging and bagging. Check your own year's syllabus before the exam.
What are the three kinds of pollination?
NCERT's chapter on sexual reproduction in flowering plants sorts pollination by where the pollen comes from.
Autogamy: pollen reaches the stigma of the same flower.
Geitonogamy: pollen reaches another flower on the same plant. NCERT notes it is functionally cross-pollination, since an agent carries it, but genetically it is like autogamy.
Xenogamy: pollen comes from a different plant. NCERT calls it the only type that brings genetically different pollen to the stigma.
Some plants, such as Viola, Oxalis and Commelina, make two kinds of flowers: chasmogamous flowers that open, and cleistogamous flowers that never open and are "invariably autogamous". Cleistogamy gives assured seed-set even without pollinators.
Why do plants need outbreeding devices?
Most flowering plants have bisexual flowers, so their own pollen easily reaches their own stigma. NCERT says continued self-pollination results in inbreeding depression, and that plants have evolved devices to discourage self-pollination and encourage cross-pollination. It describes four:
Pollen release and stigma receptivity are not synchronised. Pollen is shed before the stigma is ready, or the stigma is ready long before pollen is shed. (These two patterns are often called protandry and protogyny.)
Anther and stigma are placed at different positions, so pollen cannot reach the stigma of the same flower.
Self-incompatibility: a genetic mechanism that stops self-pollen, from the same flower or other flowers of the same plant, by inhibiting pollen germination or pollen tube growth in the pistil.
Unisexual flowers. Monoecious plants such as castor and maize carry male and female flowers on one plant: autogamy is prevented, geitonogamy is not. Dioecious plants such as papaya carry them on separate plants: both are prevented.
NCERT credits devices 1 and 2 with preventing autogamy. Only self-incompatibility and dioecy also block pollen from other flowers of the same plant.
How does self-incompatibility work?
NCERT keeps it to one line: the pistil recognises self-pollen and rejects it. A review in the Annual Review of Plant Biology by Takayama and Isogai (2005) explains that in many species this self/non-self recognition is controlled by a single, highly variable S-locus, and that self-incompatibility is "not one system, but a collection of divergent mechanisms". In the cabbage family, a pollen protein binds a receptor on the stigma; in the potato family, a ribonuclease is involved; in poppies, a calcium-dependent signal ends in the death of the incompatible pollen. You will not be examined on these details, but they show why NCERT calls it a genetic mechanism rather than a structural one.
Worked example: which pollination can each plant receive?
Maize (monoecious). Its flowers are unisexual, so autogamy is impossible. Pollen from a male flower on the same plant can reach a female flower: geitonogamy is possible, and so is xenogamy.
Papaya (dioecious). A female plant has no pollen of its own, so only xenogamy can pollinate it.
A self-incompatible plant with bisexual flowers. Self-pollen may land on its stigma by autogamy or geitonogamy, but the pistil stops it germinating or growing, so only pollen from other plants fertilises the ovules.
A cleistogamous flower of Viola. It never opens, so only autogamy is possible.
Now the breeder's view, also from NCERT. To cross a bisexual female parent, the breeder removes the anthers before they dehisce (emasculation), then covers the flower with a bag, usually butter paper. When the stigma is receptive, pollen from the chosen male parent is dusted on and the flower is rebagged. A unisexual female flower needs bagging only, because it has no anthers to remove.
Which mistakes cost marks?
Saying monoecy prevents geitonogamy. It does not; only dioecy does.
Calling geitonogamy genetically cross-pollination. It is functionally cross, genetically self.
Calling self-incompatibility a structural device. It is genetic.
Emasculating unisexual flowers. Not needed; bag them before they open.
Thinking cleistogamy is an outbreeding device. It ensures self-pollination.
To understand what happens after compatible pollen lands, read our post on pollen germination on a slide, and for the whole chapter, sexual reproduction in flowering plants. If the three kinds of pollination still blur together, the Fermi Concept Library covers every concept in biology with its own lesson, so you can revise each one separately.
A 10-minute practice routine
Draw a two-column table in your notebook: "stops autogamy" and "stops geitonogamy".
Place each of the four devices in it from memory, then check against NCERT.
Write one example plant for each kind of pollination and each device.
Make three statements with one word wrong, such as "monoecy prevents geitonogamy", and correct them.
Tomorrow, explain the maize and papaya example aloud in under a minute, without notes.
