Five Fact Friday: Plants and Smell

1. Believe it or not - plants can 'smell'. That is, they can respond to volatile chemicals in the air. A plant senses a smell when the chemicals blow over the leaves. The plant inhales the chemicals through the stomata on the leaves.
2. One common thing that plants use 'smell' for is to know when to ripen fruit or as a device to drop their leaves in the autumn. The chemical that generates this response is called ethylene. An old example of this in action is when people put a hard fruit, such as an avocado, in a paper bag with a ripe banana. The ethylene produced by the ripe banana acts to soften up the avocado.
3. Research, primarily by Dr. Consuelo De Moraes, has shown that plants such as the parasitic dodder vine use smell to find a plant to parasitise. In trial she found that the dodder would grow towards the tomato plant. In fact, when the chemicals that make the smell of the tomato plant were isolated and put on a piece of cotton, the dodder would even grow towards that. The chemical that the dodder likes so much is beta-myrcene. More information can be read in "What a plant knows" - information in the references below.
4. Plants also use smell for defence. This was originally discovered by David Rhoades and Gordon Orians, but many experiments have been done since. The experiements show that the chemicals are given off when a leaf is being damaged by a herbivore or a bacteria to let other leaves on the plant know that it needs to increase chemicals to repel the herbivore or to attack the disease. If another plant is close enough it will also 'smell' the chemicals and increase its' defence.
5. Plants also use smells to attract animals. Some plants like the bluebell has smells that we find utterly desirable and when possible we will spread them throughout the world - primarily by planting them in our gardens. Others, such as the titan arum produce the smell of rotting flesh - it even heats itself to 30 degrees centigrade to ensure that this smell travels far and wide. Why does it do this? It does it to encourage its' pollinator - tiny sweat bees.

References
Chamovitz, D. (2012) What a Plant Knows, Oxford, Oneworld Book.