Which part of the henna plant is used as a hair dye?

Henna is a dye prepared from the plant 'Lawsonia inermis', and one of the only two species of the genus 'Lawsonia', with the other being 'Lawsonia odorata'.

To make henna hair dye, the leaves of the 'lawsonia inermis' are harvested and ground into fine, richly pigmented powder. This powder is then mixed with liquid to create a thick, mud-like paste which imparts a deep stain on hair.

Henna has been used since antiquity in ancient Egypt, ancient Near East and then the Indian subcontinent to dye skin, hair and fingernails, as well as fabrics including silk, wool, and leather. Historically, henna was used in West Asia including the Arabian Peninsula and in Carthage, other parts of North Africa, West Africa, Central Africa, the Horn of Africa and the Indian subcontinent.

Commercially packaged henna, intended for use as a cosmetic hair dye, originated in ancient Egypt and the ancient Near East and is now popular in many countries in South Asia, Europe, Australia, and North America. The color that results from dyeing with henna depends on the original color of the hair, as well as the quality of the henna, and can range from orange to auburn to burgundy. Henna can be mixed with other natural hair dyes, including 'Cassia obovata' for lighter shades of red or even blond and indigo to achieve brown and black shades.