From PubMed:
http://www.pubmedcentral.nih.gov/pic...6&blobtype=pdf (NB: the link is to a .pdf-file, size 2.5 mb).
It is generally recognized that vitamin B12 is synthesized only by microorganisms. The vitamin has been identified in root nodules of leguminous plants and in roots of nonleguminous species, but in these cases the synthesis has been attributed to the nitrogen-fixing bacteria in nodules and to soil microorganisms respectively. Even though cobalt has been recognized as a micronutrient for animals and certain microorganisms, until recently there has been no conclusive evidence of its essentiality for higher plants.
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