Flowers, pollination and double fertilisation

1Saved seed: true or false?

A farmer who saves bean seed from this season's crop can plant it next season and get the same variety again, year after year. A farmer who saves seed from a field of hybrid maize and plants it gets a weaker, uneven crop. Both are flowering plants making seeds by sexual reproduction. The difference lies in their flowers: bean flowers are bisexual and pollinate themselves, usually before they even open, while maize has separate male and female flowers and is pollinated by wind, mostly with pollen from other plants.

In S3 you learned the parts of a flower and the basic steps of pollination and fertilisation. This lesson goes deeper: how flowers are classified, how their structure suits wind or animal pollination, how plants promote cross-pollination, and what happens inside the ovule during double fertilisation.

2Structure of a flower

3Types of flowers

4Pollination by insects and by wind

5Self-pollination and cross-pollination

Self-pollination is reliable, since no pollinator or second plant is needed, and it keeps a good variety true, but it reduces variation and can lead to inbreeding depression (weaker offspring that inherit two copies of harmful recessive alleles). Cross-pollination produces more variable, often more vigorous offspring, but depends on wind or animals. Many plants have features that promote cross-pollination:

• Dioecy: male and female flowers on different plants, so self-pollination is impossible.
• Dichogamy: anthers and stigmas mature at different times. In protandry the anthers shed pollen before the stigmas of the same flower are receptive (sunflower); in protogyny the stigmas are receptive first. In maize, the tassels usually start shedding pollen a few days before the silks of the same plant appear.
• Self-incompatibility: a genetic system that stops pollen from germinating, or its tube from growing, on a stigma of the same plant (or of a plant carrying the same incompatibility allele).
• Structural separation: the stigma is held above or away from the anthers, or flowers come in two forms with long and short styles (heterostyly).

6From pollen grain to double fertilisation

Each pollen grain is a male gametophyte (Lesson 3) containing a tube nucleus and a generative cell. Inside each ovule, the female gametophyte is the embryo sac: seven cells with eight haploid nuclei.

1. A compatible pollen grain lands on the stigma, absorbs water and sugars from it, and germinates: a pollen tube grows out through a pore in the wall.
2. The tube grows down through the style, secreting enzymes (such as cutinases and pectinases) that digest a path, guided by chemical signals towards an ovule. The tube nucleus stays near its tip.
3. The generative cell divides by mitosis to form two male nuclei (male gametes), which travel down the tube.
4. The tube enters the ovule through the micropyle, reaches the embryo sac, enters a synergid and bursts, releasing the two male nuclei.
5. One male nucleus fuses with the egg nucleus to form a diploid zygote ($n+n=2nn\; +\; n\; =\; 2n$), which becomes the embryo.
6. The other fuses with the two polar nuclei to form a triploid endosperm nucleus ($n+n+n=3nn\; +\; n\; +\; n\; =\; 3n$), which divides to form the endosperm, the food store for the embryo.

Double fertilisation is found only in flowering plants. Its advantage is economy: the endosperm develops only in ovules that have been fertilised, so the plant does not waste food on ovules that will never become seeds.