Legumes -

These are a special type of plant which can be trees or herbaceous. They contain nitrogen fixing bacteria in their roots which allow them to inhabit soils that are low in nitrogen. Plants can not live in soils that do not contain nitrogen as they require nitrogen to synthesize proteins. Legumes can live in such environments as the bacteria in their roots will allow then to use nitrogen from the air (once it has been fixed in the soil as nitrates). This gives them an advantage over plants without nitrogen fixing bacteria (such as Rhizobuim in clover) in their roots.