Then there is the matter of which part of the plant is used. THC tends to be more concentrated in the leaves of the plant, while CBD in its stem and seeds. It should be noted that these aspects are relative. Some degree of agreement exists that for the purest CBD, the stalk of a hemp plant (varieties of cannabis generally grown for fiber manufacturing, low in THC), or much less often the seeds. Taking into account the fact that CBD supplements are usually in an oil form, one may fathom the origins of the nearly ubiquitous hemp oil dietary supplements.