Senna (from Arabic sanā), the sennas, is a large genus of flowering plants in the family Fabaceae, subfamily Caesalpinioideae. This diverse genus is native throughout the tropics, with a small number of species reaching into temperate regions. The number of species is usually estimated to be about 300.

Sennas have for millennia played a major role in herbalism and folk medicine. Alexandrian Senna (S. alexandrina) was and still is a significant item of trans-national trade e.g. by the Ababdeh people and grown commercially, traditionally along the middle Nile but more generally in many regions around the northwestern Indian Ocean.

Sennas act as purgatives and are similar to aloe and rhubarb in having as active ingredients anthraquinone derivatives and their glucosides. The latter are called sennosides or senna glycosides. Senna alexandrina is used in modern medicine as a laxative acting on the lower bowel, it is especially useful in alleviating constipation. It increases the peristaltic movements of the colon by irritating the colonic mucosa. The plants are most often prepared as an infusion. Senna is also the primary ingredient found in most "dieter's teas". The combination of acting as a stimulant which reduces a dieter's appetite, and the laxative properties that cause food to move through their system before as many calories can be absorbed is a combination that can lead to rapid and even dangerous weight loss.

Formal Chemical Name (IUPAC)
(9R,9'R)-4,4'-dihydroxy-10,10'-dioxo-5,5'-bis(((2S,3R,4S,5S,6R)-3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran-2-yl)oxy)-9,9',10,10'-tetrahydro-[9,9'-bianthracene]-2,2'-dicarboxylic acid

References

http://en.wikipedia.org/wiki/Senna_(plant)