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Dodder on heather and gorse.
One day, far out on the moors, Flora introduced them to a curious parasitic
plant, showing them ‘a patch of heather which, from some short distance,
looked stunted and blighted and had a reddish tinge. When closely examined,
every individual plant was seen to be netted and dragged down to earth
by thin, red, threadlike runners.’ The plant is called dodder, and Flora
declared that, ‘if she were a novelist, she would write a book with that
title. It would be the story of a man or woman - she thought a woman -
of fine, sensitive nature, bound by some tie - probably marriage - to
one of a nature which was strong, coarse and encroaching, and would tell
how in time the heather person shrank and withered, while the dodder one
fattened and prospered.’
The parasitic vine called dodder is the sniffer-dog of the vegetable world. It contains almost no chlorophyll - the pigment that most plants use to make food - so to eat it must suck the sugary sap from other plants. Dodder uses olfaction to hunt down its quarry. It can distinguish potential victims from their smell, homing in on its favourites and also using scents emitted by unhealthy specimens to avoid them (Science, vol 313, p.1964)
Dodder is exceptionally sensitive to odours, but all plants have a sense of smell. In animals, sensors in the nose recognise and bind with molecules in the air. Plants also have receptors that respond to volatile chemicals.