Fertilization
The events occurring during fertilization are seen in this diagram of a flowering plant's lifecycle
Image from http://zygote.swarthmore.edu/phyto1.html
The pollen grain is deposited on the stigma (this is pollination) and germinates to produce a pollen tube.
The pollen tube grows down through the style, this growth is controlled by the tube nucleus.
The pollen grain is able to penetrate the style because of the secretion of digestive enzymes
The pollen tube enters the micropyle (by this time the generative nucleus has undergone its mitotic division so there are two male nuclei [gametes] present)
The male nuclei enter the embryo sac
- one fuses with the egg cell to form a diploid zygote - this will give rise to the embryo
- the other fuses with the two polar nuclei to form a triploid endosperm nucleus - this will give rise to the endosperm that will nourish the developing embryo.
This process is known as a double fertilization because two fusions occur.
Pollination - and adaptations for wind and insect pollination
Pollination is the transfer of the male pollen grains to the female stigma
Depending on the species this can be:
- self-pollination, where pollen is transferred to a stigma on the same plant (this obviously reduces genetic variability)
- cross-pollination where pollen is transferred to a stigma on a different plant
Pollination can be brought about by:
- wind-pollination - where the pollen are blown around and a small fraction land on a stigma
- insect-pollination - where the pollen are attached to an insect which then releases them on to the stigma of another flower
Typical characteristics of wind- and insect-pollinated plants include:
| Feature | Wind-pollinated | Insect-pollinated |
| position of flowers | above leaves | above leaves |
| petals | small, inconspicuous or absent | large, conspicuous, brightly coloured |
| nectaries | absent | present |
| scent | not scented | scented |
| stamens | pendulous (hang outside flower) | inside flower |
| anthers | move freely | fixed - positioned to come into contact with insect |
| pollen | large quantities, light, smooth grains | fewer produced, not smooth to aid attachment to insect |
| stigma | large, often feathery, hang outside flower | small, within flower, positioned to come into contact with insect |
Mechanisms for ensuring cross-pollination; protandry, protogyny and dioecious plants
Self-fertilization (as a result of self-pollination) is of value to uncommon or widely dispersed species where the chances of successful cross-fertilization (through self-pollination) are low.
However self-fertilization results in a species with limited genetic variation which is therefore at greater risk should the environment change.
Cross-fertilization (or outbreeding) keeps the degree of genetic variation in a species high and there are a number of mechanisms by which cross-fertilization is favoured.
Most plants are hermaphrodite with male and female structures in the same flower (as described in the plant reproduction pages here).
But some plants have separate male and female flowers
Some of these type of plant are dioecious - with male flowers and females flowers on different individuals. Clearly inbreeding is impossible here.
Some are monoecious - male and female flowers are on the same plant but there are other methods of preventing self-fertilization such as the two flowers being produced at different times.
Among hermaphrodite flowers self-fertilization can be prevented by having the smale and female structures develop at different times. This is known as dichogamy.
Dichogamous plants can be named according to which sexual structure develops first:
- in protoandry the stamens ripen before the carpels are mature
- in protogyny the carpels mature before the stamen