The Picture you see above is my hand with a Mehndi design. "Mendhi" is a Sanskrit word referring to a plant, Lawsonia inermis. Henna, as many people refer to it as, is actually derived from an Arabic word, pronounced Hinna for the same plant, Lawsonia inermis. This plant does not grow in colder climates. It is grown only in tropical environments.
The Mehndi (or Henna) is a dye molecule, "lawsone" which is present mostly in the leaves of the plant.
"Lawsone" has a high affinity for protein, and is used for hair dyes, and for body art. The trick is getting the paste into just the right consistency and of course the actual art of creating a design as you go along.
Mehndi is a temporary tatoo which lasts for about 2-3 weeks depending on the amount of dye used.