9.1 Essential ideas
Every plant that makes flowers or fruits reproduces sexually, and many plants rely on animal pollinators to aid in the reproductive process, especially insects and birds. Some flowers are pollinated by wind or water, but animal pollinators account for 85% of the world’s 250 000 known flowering plant species, and it is estimated that up to one-third of our total food supply is pollinated by bees. This means that much of the diversity of flowers is probably a response of natural selection to mutualistic relationships with animal pollinators.
Male and female anatomy
Animal-pollinated flowers have conspicuous, colourful petals. They are easily distinguished from wind-pollinated plants, which tend to be white or drab in comparison.Figure 9.1.4b – Parts of a flower
The carpel is also called ‘pistil’ in some textbooks – be consistent in your nomenclature
The diagram of an idealized flower shows the relative locations of the male reproductive structures (stamen consisting of anther and filament) and the female reproductive structures (carpel consisting of stigma, style and ovary). The non-reproductive structures include the sepal and receptacle. Practise drawing this diagram, paying attention to the relative size and location of the different structures.
Pollination, fertilisation and seed dispersal
Don’t confuse pollination – the process of transferring pollen from the anther on one plant to the stigma of another – with fertilisation, which is the fusion of a pollen nucleus with an ovule nucleus inside the ovary. Pollen is extremely light: it tends to stick to bristly insect exoskeleton and is deposited on nearby flowers as the insect moves in search of nectar. Pollen and ovules are the haploid gametes of sexually reproducing flowering plants which fuse to form a diploid embryo.
The diploid embryo develops into a seed. Seed dispersal occurs by wind or water, and in angiosperms via the production of a fruit deep in the ovary. Mammals and birds can disperse seeds over very long distances, as the non-digestible remains of the fruit are excreted in faeces.
Control of flowering
Flowering occurs when there is a change of gene expression in the shoot apex, involving a specific protein called phytochrome. Phytochrome is sensitive to light at different wavelengths, and exists in two reversible forms: Pfr (far-red light absorbing) and Pr (red light absorbing). During the day, both forms of the pigment are found in plants at a relatively constant concentration, but overnight, some of the Pfr reverts to Pr. The shift in equilibrium signals to the plant that it is night.
In long-day plants, flowering occurs only when the concentration of Pfr stays above a critical threshold, or when the night length is shorter than the critical night length. Some examples of long-day plants include spinach and lettuce, carnations and clovers. In short-day plants, the critical night length must be exceeded in order for flowering to occur.Figure 9.1.4c – Short-day and long-day plants
Short-day plants should be called long-night plants, because it is the night length that determines whether the plant will flower or not. This is because Pfr inhibits flowering in short-day plants, so the night length needs to be long enough to allow the phytochrome to revert to Pr. Some examples of short-day plants include chrysanthemums and poinsettas.
A physiological response to a change in day length is called photoperiodism. The control of flowering is an example.
The formation of a seed is the last stage in the reproduction of plants. Plants invest a lot of metabolic energy in developing elaborate flowers and fruits in order to attract animal pollinators and dispersers.Figure 9.1.4d – The structure of seeds
Seeds can stay dormant for long periods of time and are able to withstand environmental extremes. In order for a seed to germinate and develop shoots, optimal abiotic environmental conditions are necessary:
- water – to activate the metabolic enzymes
- appropriate temperature – for proper functioning of enzymes
- oxygen – for aerobic respiration.
Germination occurs in a stereotyped series of events:
- Imbibition – water is absorbed and swelling of the seed cracks the testa.
- The embryo releases giberellin, a hormone that signals the synthesis of amylase in the aleurone layer.
- Amylase hydrolises starches in the endosperm, breaking them up into smaller sugar molecules.
- The sugars are consumed aerobically to grow the radicles (root and shoot).
Figure 9.1.4f – Germination process
Did you know?
- Plants tend not to self-fertilise.
- Changing leaf colour is another example of photoperiodism.
- Horticulturalists can manipulate the day length in greenhouses to grow flowers all year round.
Food for thought
Germination does not require light, since energy for the process is stored in starch. That’s why bean sprouts are white, not green!
In the lab
Investigate factors that affect the rate of respiration in germinating seeds. See: Page 9.2.3
Read more about how global food supply is at risk because of dwindling populations of bee pollinators in Packer, L. (2010) Keeping the Bees: Why all bees are at risk and what we can do to save them. Toronto: Harper Collins.