SYMBIOSIS IS AN ASSOCIATION BETWEEN TWO OR MORE ORGANISMS THAT LIVE TOGETHER IN CLOSE PHYSIOLOGICAL UNION FOR MUTUAL BENEFIT. IN THIS DEFINATION, SYMBIOSIS REFERS TO THE INTIMATE ASSOCIATION OF TWO DIFFERENT KINDS OF ORGANISMS.
THE IMPORTANCE OF NITROGEN FIXING BACTERIA, RHIZOID ASSOCIATED WITH LEGUMES IS WELL KNOWN. ROOTLETS OF LEGUMES HAVE SWELLINGS,INHABITED BY BACTERIA. HERE, THEY DERIVE CARBOHYDRATES FROM THE HOST AND IN TURN THEY FIX THE ATMOSPHERIC NITROGEN IN THE ROOTS. THIS NITROGEN IS USED BY PLANTS.
THE WOOD BORING BEETLES HAVE AN ELABORATE APPARATUS ON THEIR HEADS FOR CARRYING THE FUNGUS. THE FEMALES OF THESE SPECIES HAVE AN APPARATUS CONNECTED WITH OVIPOSITOR WHICH COATS THEIR EGGS WITH FUNGAL SPORES AS THEY ARE LAID. IN ALL THESE ASSOCIATIONS, THE FUNGUS PROVIDES FOOD AND SHELTER, AND IT THRIVES ON THE EXCRETORY MATERIAL OF ITS HOST.
MUTUALISM AND SYMBIOSIS ARE THE TERMS APPLIED TO VERY SIMILAR TYPES OF ASSOCIATIONS WHERE, ONE DERIVES NUTRITION FROM ANOTHER AND IN TURN BENEFITS THE FOOD SUPPLIER IN SOME OTHER WAY. FOR EXAMPLE , IN LICHENS THE ALGAE PROVIDES PART OF FOOD FOR CONSUMPTION BY THE FUNGAL COMPONENT. IN TURN THE FUNGUS PROVIDES PROTECTION TO THE ALGAE AGAINST DESICCATION. MANY OF THESE RELATIONSHIPS AND TERMINOLOGIES OF BIOTIC COMPONENTS ARE BASED ON FOOD HABITS.
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