|
In ancient Greece, the person getting free meals for entertaining others by his amusing converstions was called a parasite - 'para' refers to beside and 'sitos' refers to grains. Thus, parasitism is defined as an association between individuals of two different species which is beneficial to one and generally harmful to another.
The benefiting partner is called the parasite and the other partner is called the host. The parasite is dependent on the host for food or shelter or both.
Parasites may be ectoparasites, that which live on the outer surface of the host (ticks, mites, leeches) or may be endoparasites, that which live inside the body of the host (tapeworm, liver fluke).
Parasites are also classified as obligate or facultative. Obligate parasites have to live parasitically at all times.
For example: Phytophthora, a fungus which causes potato blight.
Facultative parasites may feed parasitically or saprophytically.
For example: Candida (fungus causing thrush in humans) or Pythium (fungus-like organism causing damping-off in seedlings)
Most parasites are microbes like the bacteria and viruses. The latter require the host cell for replication. For example, tobacco mosaic virus which infects only the tobacco plants.
Among the higher organisms, most parasites are animals. However, there are certain plants which lack chlorophyll pigment (without which photosynthesis cannot take place) and are thus parasites on other plants. For example, Cuscuta (Dodder), Viscum.
As can be seen in the diagram, Cuscuta lacks leaves and the roots are modified into specialised structures called haustoria. Haustoria penetrate the stem of the host and draws nutrition from the vascular system of the host directly.
Activities/ Experiments
a. What do you find after 2 to 3 days?
There is a cotton-like black coloured growth on the bread.
b. Can this organism be called a saprophyte?
Yes, it can be as it is growing and feeding on organic matter.
c. Observe it under a microscope.
There are some other interesting forms of nutrition like the coprophagy which refers to feeding on faeces and carrion feeders which feed on decomposing animal bodies.
There are certain plants which grow in nitrogen-deficient conditions and they meet this requirement by feeding on insects. They are called the insectivorous plants. For example, pitcher plants (Nepenthes), bladderwort (Utricularia) and Venus fly trap (Dionaea). These plants have the photosynthetic mechanisms in addition to modifications for catching and digesting insects.
The above-mentioned types of nutrition divided the different organisms under three categories. They are:
- Producers
- Consumers
- Decomposers
Producers
They are the autotrophs. Their activity sustains all the organisms as they alone can trap the solar energy into food. This in turn is used by the next category.
Consumers
Those that feed directly on the plants are called herbivores and those that feed on the latter are called carnivores.
Decomposers
They are the saprophytic and saprozoic organisms that decompose the dead plant and animals before feeding on them.
| 
