A form of 'living together' in which each organism benefits the other. For example, a lichen is a symbiosis between a fungus and an alga. And the nitrogen-fixing root nodules of legumes are a symbiosis between the legume and the Rhizobium. Symbiosystem

A subsystem of an ecosystem that involves symbiosis, or cooperation, between two entirely different species of organism. Probably the most important example of symbiosis in crop husbandry is the nitrogen fixing root nodules on the roots of pulses and fodder legumes, formed by species of a bacterium called Rhizobium. Symptom

A change in the nature or appearance of a plant that is indicative of disease. Symptomless carrier

A plant, or cultivar, that has so much horizontal resistance to a virus disease that it exhibits no symptoms of that disease, in spite of being infected with it. Symptomless carriers can be a source of infection for nearby susceptible crops. However, if all cultivars were symptomless carriers, the virus disease would no longer be important.

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