Henna is Lawsonia inermis Fam. Lythraceae, a monotypic genus, the single example being L. inermis,2 native to North and East Africa, introduced and cultivated in the Persian Gulf region, the Arabian Peninsula, the Levant, and South Asia.3 It grows in semi-arid tropical zones, and tolerates dry soil and extended drought. It does not tolerate frost and thrives where temperatures are between 11C and 45C. Henna is a biennial dicotyledonous herbaceous shrub-like desert tree. A henna tree will naturally grow six to twenty feet in height, though under cultivation, the tree is pruned once or twice a year to a short bush to produce more henna leaves per hectare. The leaves are smooth, opposite, sub-sessile, elliptically-shaped and broadly lanceolate, with depressed veins clearly visible on the dorsal surface.
Merck’s Report, Volume XXIX, 1920,6 notes that nineteenth century western botanists erroneously claimed that the one species, L. alba, had two varieties, L. inermis (unarmed) and L. spinosa (with spines). This confusion arose from the fact that the young henna plants are devoid of spines while the older plants have branchlets that harden into spines: they were both the same plant at differing stages of maturity.
During the quaternary glaciation, henna would have had the best chance for survival in the semiarid frost-free areas in Africa. The single genus implies a relatively recent genetic bottleneck from henna withdrawing to a relatively small survival area during the most recent glacial period, probably along 15° N, across what is presently the Sahel. During the Neolithic Subpluvial,7 the henna growing range is likely to have expanded across the greening Sahara when it was a savannah, when Africa’s seasonal monsoons shifted slightly north.8 People living across that region probably first became familiar with henna during that period; henna was certainly used in Egypt, at the eastern edge of the region, to mask graying hair around 3000 BCE.
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Links to specific topics in this chapter:
The Botany of Henna
The Phytochemistry of Henna
Microscopy of Products Labeled as Henna in Present and Past Commercial Production
