What you'll learn
Reproduction in flowering plants covers the structure of the flower, the production of gametes, pollination, the distinctive double fertilisation of angiosperms, and the formation and dispersal of seeds and fruits. At CAPE level you must be able to relate floral structure to the method of pollination, describe the development of pollen grains and embryo sacs, explain double fertilisation and what each fusion produces, and explain the mechanisms by which plants avoid self-pollination. By the end of this topic you should be able to label and explain flower structure, compare insect and wind pollination, describe gamete formation and double fertilisation, explain seed and fruit formation, describe dispersal mechanisms, and compare sexual with asexual reproduction in plants.
Key terms and definitions
Stamen — the male part of a flower, consisting of anther and filament
Carpel — the female part of a flower, consisting of stigma, style and ovary
Anther — the structure producing pollen grains
Pollen grain — the structure containing the male gametes
Ovule — the structure within the ovary containing the embryo sac
Embryo sac — the female gametophyte within the ovule, containing the egg cell and polar nuclei
Pollination — the transfer of pollen from anther to stigma
Self-pollination — transfer of pollen to a stigma of the same plant
Cross-pollination — transfer of pollen to a stigma of a different plant of the same species
Double fertilisation — the two fusions characteristic of flowering plants
Endosperm — the triploid nutritive tissue formed by the second fusion
Micropyle — the pore in the ovule through which the pollen tube enters
Dehiscence — the splitting open of the anther to release pollen
Testa — the seed coat, formed from the integuments of the ovule
Core concepts
Flower structure
A flower is a modified shoot adapted for sexual reproduction, and its parts are arranged in whorls.
The sepals form the outermost whorl and protect the flower in bud.
The petals lie within them, and in insect-pollinated flowers are usually large, brightly coloured and scented to attract pollinators.
The stamens are the male parts. Each consists of an anther, in which pollen is produced, borne on a stalk called the filament.
The carpels are the female parts. Each consists of a stigma, which receives pollen; a style, which supports the stigma and through which the pollen tube grows; and an ovary, containing one or more ovules.
The receptacle is the swollen tip of the flower stalk to which all these parts attach.
Nectaries at the base of the petals secrete nectar in insect-pollinated flowers.
Pollen and embryo sac formation
Within the anther are four pollen sacs containing diploid pollen mother cells. Each divides by meiosis to produce four haploid cells, which develop into pollen grains. The nucleus of each then divides by mitosis to give a tube nucleus and a generative nucleus, and the generative nucleus later divides again to produce two male gametes. The grain develops a tough, sculptured outer wall, which resists desiccation and whose pattern is species-specific.
Within the ovule, a diploid megaspore mother cell divides by meiosis to give four haploid cells, three of which degenerate. The surviving cell undergoes three rounds of mitosis to produce eight haploid nuclei within the embryo sac.
These eight are organised functionally: one becomes the egg cell, two become the polar nuclei at the centre, and the remaining five occupy the ends of the sac and take no part in fertilisation. Only the egg cell and the two polar nuclei matter for what follows.
The ovule is enclosed by integuments, with a small pore called the micropyle at one end.
Pollination and floral adaptation
Pollination is the transfer of pollen from anther to stigma, and it must be distinguished from fertilisation, which is the fusion of gametes. Candidates confuse the two routinely.
Insect-pollinated flowers are adapted to attract and use animals. The petals are large, brightly coloured and often scented, and nectaries produce nectar as a reward. The anthers and stigma are enclosed within the flower, positioned so that a visiting insect brushes against them. Pollen grains are relatively few, large, and sculptured or sticky so that they adhere to the insect. The stigma is small and sticky.
Wind-pollinated flowers dispense with attraction entirely. Petals are small, green or absent, with no scent and no nectar. The anthers are large and hang outside the flower on long filaments so that pollen is readily shaken free. Pollen grains are produced in very large numbers, and are small, light and smooth so that they are carried far by air currents. The stigmas are large and feathery, hanging outside the flower to present a large surface area for catching airborne pollen.
The logic is consistent throughout: insect pollination is targeted and therefore economical in pollen but requires investment in attraction, whereas wind pollination is untargeted and therefore requires enormous quantities of pollen but no attraction at all.
Preventing self-pollination
Cross-pollination produces greater genetic variation, so many plants have mechanisms discouraging or preventing self-pollination.
Dichogamy is the maturation of anthers and stigmas at different times. In protandry the anthers mature first; in protogyny the stigmas mature first.
Spatial separation places the anthers and stigma so that pollen is unlikely to reach the stigma of the same flower, for example with the stigma held above the anthers.
Dioecious species have male and female flowers on separate plants, so self-pollination is impossible. Monoecious species have separate male and female flowers on the same plant, which reduces but does not prevent it.
Self-incompatibility is a genetic mechanism in which pollen landing on a stigma of the same plant fails to germinate, or the pollen tube fails to grow down the style, because it is recognised by the plant's own genotype.
Double fertilisation
Double fertilisation is unique to flowering plants and is the most commonly examined part of the topic.
When a compatible pollen grain lands on the stigma, it absorbs water and nutrients from the stigma surface and germinates. A pollen tube emerges and grows down through the style, its growth directed by the tube nucleus at its tip and by chemical signals from the ovule. The tube secretes hydrolytic enzymes that digest a path through the tissue of the style, and the products are absorbed as nutrients.
The two male gametes travel down the tube behind the tube nucleus.
The pollen tube enters the ovule through the micropyle, and the tube nucleus disintegrates once its function is complete.
Two separate fusions then occur, and this is what makes the process double.
One male gamete fuses with the egg cell, forming a diploid zygote, which develops into the embryo.
The other male gamete fuses with the two polar nuclei, forming a triploid nucleus, which develops into the endosperm, a nutritive tissue that supplies the developing embryo.
The ploidy of each product is frequently asked: the zygote is diploid because one haploid gamete fuses with one haploid egg, while the endosperm is triploid because one haploid gamete fuses with two haploid polar nuclei.
The advantage of double fertilisation is that nutritive tissue is produced only where fertilisation has actually occurred, so the plant does not invest resources in provisioning ovules that have not been fertilised.
Seed and fruit formation
After fertilisation, each part of the ovule and ovary develops into a corresponding part of the seed and fruit, and the correspondences are examinable.
The zygote develops into the embryo, consisting of a plumule which will form the shoot, a radicle which will form the root, and one or two cotyledons which are seed leaves.
The triploid nucleus develops into the endosperm. In endospermic seeds such as maize this tissue persists and is the food store. In non-endospermic seeds such as the broad bean, the endosperm is absorbed by the cotyledons during development, and the swollen cotyledons become the food store.
The integuments harden to form the testa, the protective seed coat, and the micropyle persists as a small pore through which water is absorbed at germination.
The ovule as a whole becomes the seed.
The ovary wall becomes the pericarp, the fruit wall.
The ovary as a whole becomes the fruit.
The seed then loses water, becoming dormant with a very low water content and a much reduced metabolic rate, which allows it to survive unfavourable conditions.
Seed and fruit dispersal
Dispersal reduces competition between the parent plant and its offspring and between the offspring themselves, for light, water, mineral ions and space, and allows colonisation of new habitats.
Wind dispersal uses fruits or seeds that are light and have a large surface area, such as the winged fruits of the mahogany or the parachute of the dandelion.
Animal dispersal operates in two ways. Succulent fruits are brightly coloured, sweet and nutritious, so that animals eat them and the resistant seeds pass through the gut unharmed and are deposited elsewhere. Hooked fruits attach to fur or feathers and are carried before dropping off.
Water dispersal uses buoyant fruits with air spaces and a waterproof outer layer, the coconut being the standard Caribbean example, able to float in sea water for long distances and germinate on a distant shore.
Explosive or mechanical dispersal uses a pod that dries unevenly and splits suddenly, flinging the seeds away, as in many legumes.
Sexual and asexual reproduction compared
Sexual reproduction involves gametes and produces genetic variation through meiosis and random fertilisation. That variation provides the raw material for natural selection, so populations can adapt if conditions change, and seeds allow dispersal and survival through unfavourable seasons. Against this, it requires a pollinating agent, is slower, and expends considerable energy on flowers, nectar and pollen.
Asexual reproduction in plants uses vegetative structures such as runners in the strawberry, tubers in the potato, bulbs in the onion and rhizomes in ginger. It is faster, requires no pollinator, and produces offspring genetically identical to a parent already proven suited to that habitat, with a substantial food store to support early growth. Against this, there is no genetic variation, so the whole population is vulnerable to a single disease or environmental change, and offspring grow close to the parent and compete with it.
Many plants use both, which allows the advantages of each.
Worked examples
Example 1: Explaining double fertilisation (5 marks)
Describe double fertilisation in a flowering plant and state the ploidy and fate of each product.
A compatible pollen grain germinates on the stigma and produces a pollen tube, which grows down through the style directed by the tube nucleus, secreting hydrolytic enzymes to digest a path. The two male gametes travel down the tube, which enters the ovule through the micropyle.
One haploid male gamete fuses with the haploid egg cell. The product is a diploid zygote, which divides by mitosis to form the embryo, consisting of plumule, radicle and cotyledons.
The other haploid male gamete fuses with the two haploid polar nuclei. The product is a triploid nucleus, which develops into the endosperm, a nutritive tissue supplying the developing embryo.
Because two separate fusions occur, the process is described as double fertilisation, and it ensures that nutritive tissue is produced only in ovules that have actually been fertilised.
Example 2: Relating structure to pollination method (5 marks)
A flower has small green petals, no scent, large anthers hanging outside the flower and large feathery stigmas. Identify the pollination method and explain three of the adaptations.
The flower is wind-pollinated.
The petals are small and green with no scent or nectar because there is no need to attract an animal pollinator, so resources are not expended on attraction.
The anthers are large and hang outside the flower on long filaments, so they are fully exposed to air currents and the pollen is readily shaken free and carried away.
The stigmas are large and feathery and hang outside the flower, presenting a very large surface area to the air so that airborne pollen grains are more likely to be caught.
In addition, such flowers produce very large quantities of small, light, smooth pollen, because wind dispersal is untargeted and the probability of any individual grain reaching a stigma of the same species is very low.
Example 3: Explaining the value of dispersal (4 marks)
Explain why dispersal of seeds away from the parent plant is advantageous.
If seeds germinated directly beneath the parent, the seedlings would grow in the shade of the parent and would compete with it, and with each other, for light, water, mineral ions and space. Many would fail to establish.
Dispersal separates the seedlings from the parent and from one another, reducing this competition so that a greater proportion survive.
It also allows colonisation of new habitats, which may offer more favourable conditions or fewer competitors, and it extends the range of the species.
Spreading the offspring over a wider area additionally reduces the chance that a single localised event, such as a disease outbreak or a fire, destroys all of them.
Common mistakes and how to avoid them
The most frequent error is confusing pollination with fertilisation. Pollination is the transfer of pollen to a stigma; fertilisation is the fusion of gametes, and it occurs after the pollen tube has grown down the style.
Students often state that the endosperm is diploid. It is triploid, because one haploid gamete fuses with two haploid polar nuclei.
Another common slip is saying that the ovule becomes the fruit. The ovule becomes the seed; the ovary becomes the fruit.
Many candidates describe wind-pollinated flowers as having no stigma or anthers. They have both, but positioned outside the flower and shaped differently from those of insect-pollinated flowers.
In adaptation questions, answers frequently list features without giving the functional reason. Feathery stigmas earn the mark only when linked to catching airborne pollen.
Finally, candidates often describe asexual reproduction as producing variation. It produces genetically identical offspring, which is precisely its main disadvantage.
Exam technique for "Reproduction in flowering plants"
Learn the developmental correspondences as a list — zygote to embryo, triploid nucleus to endosperm, integuments to testa, ovule to seed, ovary wall to pericarp, ovary to fruit — since questions frequently ask for several at once.
State ploidy wherever a nucleus or cell is named. Haploid gametes, diploid zygote and triploid endosperm are all commonly credited.
For pollination comparisons, use the same headings for both methods — petals, scent, nectar, anther position, pollen quantity and form, stigma form — so the comparison is parallel.
When explaining an adaptation, always take it through to the consequence for pollination, dispersal or survival.
Keep the terms pollination, fertilisation and germination distinct, and use each only for the process it names.
Quick revision summary
A flower consists of sepals, petals, stamens with anther and filament, and carpels with stigma, style and ovary containing ovules. Pollen mother cells divide by meiosis to give pollen grains containing a tube nucleus and two male gametes, while in the ovule a megaspore mother cell gives rise to an embryo sac containing an egg cell and two polar nuclei. Insect-pollinated flowers have large scented coloured petals, nectar, enclosed anthers and stigma, and few large sticky pollen grains; wind-pollinated flowers have small green petals, no scent or nectar, exposed anthers, feathery stigmas and abundant small smooth pollen. Self-pollination is limited by dichogamy, spatial separation, dioecy and self-incompatibility. In double fertilisation, the pollen tube grows down the style and enters the micropyle, one male gamete fusing with the egg to give a diploid zygote and the other with two polar nuclei to give triploid endosperm. The zygote becomes the embryo of plumule, radicle and cotyledons, the integuments become the testa, the ovule becomes the seed, and the ovary becomes the fruit. Dispersal by wind, animals, water or explosive mechanisms reduces competition and allows colonisation.