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Senna is a large genus of about 250–260 species 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.
Typically Senna species have yellow flowers. Some species of Senna are notable for being host to particular butterfly species — for instance Cloudless Sulphur butterflies.
Senna alexandrina is a small shrub, about 0.5-1 m high, with a pale green smooth erect stem, long spreading branches, bearing four or five pairs of leaves. The flowers are small and yellow, the pods broadly oblong and containing about six seeds.
Senna is an Arabian name, and the plant is grown mostly in Nubia. Twice a year the plants are cut down, dried in the sun, stripped and packed in palm-leaf bags and sent on camels to Essouan and Darao then up the Nile to Cairo or else to Red Sea ports.
It is a purgative, similar to aloe and rhubarb in having as active ingredients anthraquinone derivatives and their glucosides. Its action is on the lower bowel, and is especially useful in alleviating constipation. It increases the peristaltic movements of the colon.
Another species of senna, Cassia obovata, is used as a hair treatment with effects similar to henna, but without the red color. The active component is an anthraquinone derivative called chrysophanic acid, which is also found in higher concentrations in rhubarb root. It adds a slight yellow color. Cassia obovata is often called “neutral henna”.