EtymologyAlthough the spelling "marijuana" is more common in current usage, the correct spelling in the Marihuana Tax Act is "marihuana". "Marihuana" was the spelling most commonly used in Federal Government documents at the time. To stay consistent with prior law, it is still spelled "marihuana" in some congressional bills such as HR 3037, the Industrial Hemp Farming Act of 2005.
In addition, the Marihuana Tax Act of 1937 legitimized the use of the term "marijuana" as a label for hemp and cannabis plants and products in the US and around the world. Prior to 1937, "marijuana/marijuana" was slang; it was not included in any official dictionaries.[19] The slang word marijuana/marijuana is probably of Mexican origin .....
the link below can give more information on this word.
http://en.wikipedia.org/wiki/Marihuana_Tax_Act_of_1937