Thursday, October 30, 2008

Parasites and Saprophytes

These are plants which depend entirely or partly on other organisms. We have already dealt with the fungi, which are the largest group of parasites and saprophytes in the plant kingdom. But there are other plants which exist on living material (parasites) or make use of dead organic substances (saprophytes) Parasites may be total or partial that is they either get the whole of their food materials from the host plant on which they live, and have no green parts, or they have chlorophyll of their own and can manufacture a certain amount of their own food, but also need some from the host.

Dodder, in the Convolvulus family is a total parasite and can be seen on the stems of plants such as gorse and heather. This is a stem parasite but other plants such as broomrape are root parasites and penetrate and live on the roots of a host plant.

Mistletoe is another good example of a parasite which occurs on the stems and trunks of trees, while several members of the foxglove family live on the roots of grasses in meadows. Saprophytes make use of decaying matter and usually occur in woodlands where there is plenty of rotting vegetation. The bird's nest orchid can be seen growing in woods and is a brownish plant which only appears above ground when it sends up a flowering stem. The roots actually live in association with a fungus which breaks down the rooting materials it lives on into soluble substances. The fungus also penetrates the roots of the orchid and gains some of its materials from the cells there, while the orchid gets its supplies of food materials through the fungus. Thus the two plants are living together and depending on each other for their existence, this being called symbiosis.

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