FAMILY: Fabaceae — Legume family
SYNONYMS: Cassia lindheimeriana
ETYMOLOGY: The name of the genus Senna is taken from the Arabic sana, a word used for cassia plants in general.
The species name lindheimeriana honors Ferdinand Jacob Lindheimer (1801–1879), a German emigre and plant collector who is considered to be the “father of Texas botany”.
GEOGRAPHIC RANGE: From southeastern AZ, southern NM, western Texas into Mexico
HABITAT: Found on gravelly flats and hills, grasslands, canyons, and washes, 3,000-5,500'
BLOOMING SEASON: May to October
DESCRIPTION: Perennial plants, 3-6' tall with a spread of 2-3'. The herbaceous stems grow from a woody root crown.
Pinnately compound leaves grow to 6" long and have 4-8 pairs of leaflets, each 1-2" long. Leaflets are oblong and rounded with fine hairs, particularly on the undersides. Fine hairs on the leaf margins give them a silvery appearance. Stems are also covered with fine hairs.
Five to twenty flowers are held on short racemes that grow from leaf axils near the tip of the stems. The sepals have fine hairs, and the petals have prominent veination. One-inch diameter flowers are bright yellow to yellow orange. The flower has 9-10 stamens of which only 6-8 are fertile and contain pollen. Flat brown seed pods, 2" long and 1/4" wide, are covered with fine hairs that give them a velvety feel.
Pollination—Plants in the genus Senna have a specialized method for dispersing pollen. Typically, a flower’s anthers split lengthwise to release pollen, which is easily gathered by bees. The senna’s anthers have only a small pore at the top through which the pollen can be released, and in order to get to the pollen, the bee must vibrate the stamen and shake the pollen out through an apical pore. This "buzz pollination" technique is limited to bumble bees, carpenter bees, some sweat bees, and digger bees.
Protection—Another unusual plant-insect interaction involves extrafloral nectaries, nectar-secreting glands located between each pair of compound leaves. These glands are tubular stalks with glistening tips that exude nectar to attracts ants. The ants then protect the plant by eating any insects that would feed on the leaves or flowers.
Desert Adaptations: The fine hairs covering the leaves and stem of the plant deflect sunlight to lower the temperature of the plant.
Toxicity: Plants are fatally poisonous to livestock. When brewed into a tea, it has been used medicinally as a strong laxative.