Tannins are found primarily in the bark, leaves and immature fruits of a wide range of plants.  They form complexes with proteins and the cellulose in plants--in other words, tannins are known to link up with other proteins  Tannins are polyphenolic compounds that bind to and precipitate proteins.  In nature, tannins are a self-defense mechanism.  They have an astringent, and putting off taste that dissuades or repels pests and herbivores.  If an animal or insect begins to chew on the plant tissue, the tannins are released from the cellular compartments and start binding with the proteins and other cell components, making them taste unpleasant and indigestible.  

The henna plant requires arid and hot climates in order to grow and thrive.  Just a couple of rainfalls per season is enough watering for the plant to survive.  With its natural defense mechanism of tannins, it does not attract pests, thus, making it easy to grow the plant organically.  Because of its low maintenance, low to no requirement of pesticides, and low water requirement, the henna plant is farmed by many low-income and poor farmers. 

The henna plant, lawsonia inermis, contains tannins, which is what binds to keratin.  This includes the skin, hair, leather and fibers such as cotton or silk.  The henna plant contains lawsone (a-hydroxynaphthoqunone), various phenolic glycosides, coumarins, xanthones, quinoids, B-sitosterol glycoside, flavonides, fats, resin and tannins.  

In an analysis of air dried leaf powder, the following was determined:  
Moisture content:  9%
ash content:  14.85%
tannin content 10.2%
water soluble matter 22-33%

(Reference: Natural Excipients, by By Dr. R. S. Guad, Dr. S. J. Surana, Dr. G. S. Talele, Mrs. S. G. Talele, Mr. S. B. Gokhale.  2006)