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Parasitic relationships

A parasite lives either in or on the body of another organism called the host.

Plant parasites

Facts logo Johnny's Fact File No.64
More than 1% of flowering plants are parasitic on other plants. This means that there are around 3000 species of parasitic plants.

Dutch elm disease is caused by a fungus, Ceratocystis ulme. This parasite spread from Asia to Europe at the turn of the century and reached the United States of America in the 1930's.

An elm tree © Paul Billiet

The fungus is spread from tree to tree by the elm bark beetle, Scolytus multistriatus. The female beetle digs a chamber in an infected tree where she lays her eggs. The fungus grows in the chamber where it produces its spores. When the eggs hatch, the larvae feed on these spores. When the young beetles leave the infected elm tree, they carry spores to other elm trees. As the beetles move over the younger, growing parts of the healthy elm tree, some spores are left behind. These spores germinate and grow in the water carrying vessels of the elm from where the fungus spreads throughout the tree.

An elm bark beetle © Paul Billiet

The fungus grows down into the roots, blocking the vessels which carry water through the plant. Eventually the roots die and the tree falls over. The first signs of Dutch elm disease are leaves on some of the branches curling up and turning brown. These leaves fall from the tree before the autumn.

Galleries inside an elm tree which were made by the elm bark beetle © Paul Billiet

The disease can kill a young tree very quickly and older trees are only able to resist for a few years. When Dutch elm disease reached the United States of America it killed 27000 elm trees in only three years.

The female bark beetle will only make her egg chamber in an infected elm because the fungal spores provide food for the larvae when they hatch. In this way the cycle is continued through the reproduction of the beetle. This is why scientists try to control the disease by preventing the beetle from feeding and reproducing. This is done by burning all the dead trees ( in order to prevent the female beetle digging her chamber) and by the use of a powerful insecticide.

Once an elm tree has been infected there can be no cure; the fungus is a parasite which kills its host.

Facts logo Johnny's Fact File No.65
The parasitic dodder is called the Devil's Thread in North America. Every year in the USA plant parasites cause 3 billion dollars worth of damage to crops.

The dodder plant does not contain any chlorophyll. Its stem and small leaves are a pink-red colour. The dodder is able to survive without chlorophyll because it lives parasitically on certain other plants, such as gorse and heather.

The dodder plant © Paul Billiet

The seed starts to germinate on the ground and produces one temporary root. The shoot grows upwards until it touches the stem of another plant. When this contact has been made, the stem of the dodder continues to grow upwards by spiralling around the host's stem. Special root-like structures grow from the stem of the dodder into the stem of the host. The root-like structures penetrate the host's tissues until they reach the vessels which carry the sugars through the host plant.

Facts logo Johnny's Fact File No.66
Mistletoes transpire ten times faster than other plants. This allows them to pull water from the host plant into there own tissues. 
The North American dwarf mistletoe causes the loss of 20 million m3 pf wood each year. It has explosive fruits which can shot out their seeds at 100km/h and project them as far as 15m from the parent plant!

The temporary root, which served as an anchor before the dodder attached itself to the host, shrivels up and rots; the dodder has no more need of it. The dodder is now a well established parasite, feeding from the products of photosynthesis made by the host.

The mistletoe is a plant parasite on many different types of trees. Unlike those of the dodder, the leaves of the mistletoe do contain chlorophyll and the plant makes its own food by photosynthesis. The mistletoe develops high up on the branches of the host tree. It develops root-like structures from its stem which penetrate the tissues of the host and take water from the host's water-carrying vessels. A mistletoe plant can be easily identified in winter because it remains green when the leaves of its host have fallen to the ground.

Mistletoe © Shirley Burchill

The mistletoe bush is usually between 0,6 and 0,9 metres in length. Its flowers develop into white berries which are poisonous to humans. The seeds are spread by seed-eating birds. The berry of the mistletoe has a sticky juice, very much like chewing gum. A bird which eats the mistletoe's berries wipes its beak on the branch of a tree and the seed is wiped off with the juice.

If the seeds are stuck to a suitable branch they will germinate, sending root-like structures into the tissues of the host. The mistletoe does not kill its host but it can cause the host to grow abnormally and decrease the host's ability to reproduce.

Human Parasites I
Human Parasites II

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