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Reproduction in Plants

12 min read

Sexual reproduction in flowering plants

The flower is the reproductive organ of an angiosperm. The stamen (anther and filament) makes pollen, which contains the male gametes. The carpel (stigma, style and ovary) holds the ovules, which contain the female gamete.

Parts of a flowerA cross-section of a flower. The male parts are the stamens: each has an anther that makes pollen on a filament. The female part is the carpel, with a stigma that catches pollen, a style, and an ovary containing ovules. Petals attract insects and sepals protect the bud.stigmastyleovaryovuleantherfilamentpetalsepalstalk

Gametogenesis

In the anther, pollen mother cells divide by meiosis to make four haploid cells, each of which becomes a pollen grain with a generative nucleus and a tube nucleus. In the ovule, one embryo-sac mother cell divides by meiosis; one cell survives and divides by mitosis three times to form an embryo sac with eight nuclei, one of which is the egg cell and two of which are the polar nuclei.

Pollination and fertilisation

Pollination is the transfer of pollen from an anther to a stigma, by insects, wind, water or animals. Self-pollination is within the same flower or plant, and cross-pollination is between different plants, which gives more variation. The pollen grain then grows a pollen tube down the style to the ovule. In double fertilisation one male nucleus fuses with the egg to form the zygote (2n) and the other fuses with the two polar nuclei to form the endosperm (3n).

Pollination and fertilisation in a flowering plantA pollen grain lands on the stigma and grows a pollen tube down the style to the ovule. Two male nuclei pass down the tube. One fuses with the egg cell to form the zygote and the other joins two polar nuclei to form the food store, the endosperm. This is double fertilisation. The ovule becomes the seed and the ovary the fruit.pollen grain on the stigmapollen tubestyleovaryegg cell + male nucleus → zygotepolar nuclei + male nucleus → endosperm
A seed and germinationA bean seed has a seed coat, two seed leaves called cotyledons that store food, a radicle that becomes the root and a plumule that becomes the shoot. With water, oxygen and warmth the seed germinates: the radicle grows down first, then the plumule grows up and the first leaves open.bean seedseed coatcotyledon (food store)radicle (root)plumule (shoot)germinating seedlingneeds water, oxygen and warmth

Life cycles and alternation of generations

Plants alternate between a haploid gametophyte that makes gametes and a diploid sporophyte that makes spores. In mosses, the gametophyte is the leafy green plant. In ferns, a small prothallus is the gametophyte and the leafy fern is the sporophyte. In flowering plants, the sporophyte is the whole plant and the gametophytes are tiny: the pollen grain and the embryo sac.

PlantDominant stageReduced stage
MossGametophyte (n)Sporophyte on the gametophyte
FernSporophyte (2n)Prothallus (n)
Flowering plantSporophyte (2n)Pollen and embryo sac (n)
Meiosis halves the chromosome number; fertilisation restores it. Here the plant has 2n = 20.
Try itSeeds from a fruit

Each ovule that is fertilised becomes a seed. Choose the ovules in the ovary and the share that are fertilised.

Check yourself The endosperm nucleus is...
Check yourself What does the ovule become after fertilisation?
Check yourself Cross-pollination is...

Key terms

Pollination
Transfer of pollen from anther to stigma.
Double fertilisation
Two fusions in the embryo sac of a flowering plant.
Gametophyte
The haploid generation that makes gametes.
Sporophyte
The diploid generation that makes spores.

Summary

  • Stamens make pollen and carpels hold ovules.
  • Meiosis in the anther and ovule makes haploid cells.
  • Double fertilisation forms the zygote and the endosperm.
  • Plants alternate between gametophyte and sporophyte generations.
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