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Senna pallida (Vahl) H.S.Irwin & Barneby 1982
pronounced: SEN-nuh PALL-ih-duh
(Fabaceae — the morning glory family)
subfamily: Caesalpinioideae – the cassia subfamily
synonym: Cassia biflora L. 1753
pronounced: KASS-ee-uh by-FLOR-uh
common name: Two-flowered Cassia
Senna is the Latin form of the Arabic word sanā for a thorny bush; pallida is from the Latin pallidus, pale. Cassia (or casia) is the Roman name of a tree with aromatic bark, perhaps the wild cinnamon. Biflora is Latin, from bis, twice, and floreo, to bloom, flower.
This spreading little shrub comes from the southern United States, Central America, and northern parts of South America, where it is found usually in brushy plains and hillsides, either moist or dry, often in pineoak forest, and along roadsides. It is usually found at low elevations, but it has been found as high as 1800 m. It is grown in other countries, including Australia, as an ornamental. The plant photographed is adjacent to the Arcadia Hotel, on the Alma Bay side.
The plant is usually between 1 and 3 m tall. It is much-branched, and its branches are slender, the stems slightly yellowish-rusty coloured, with short appressed rusty-coloured hairs.
The alternate leaves are paripinnately compound, 3 – 4 on a rachis, arising from a common point. They are stipulate, with 2 stipules, linear, pointed, brown-purple in colour, the rachis grooved on the dorsal side. The leaves are very small, mostly 2 – 3 cm long, and the leaflets are tiny, well under 1 cm in length. There are 6 – 10 pairs of opposite leaflets, arising from a raised rounded collar, broadly oblong or obovate-oblong, with very obtuse apices that are mucronulate. The leaflets are minutely hairy. On the rachis of each compound leaf, between the bottom-most leaflets (those next to the woody stem), there is a conspicuous conical gland, which is thought to produce a substance that attracts ants, that attack any animal trying to eat the leaves.
The flowers usually occur in pairs (but there can be up to 4), hence the common name, and the specific of the synonym. They are quite large (up to about 4 cm across), a golden yellow in colour. The peduncles are short and softly hairy. There are 5 petals, 5 sepals, polysepalous, and 7 stamens, polyandrous. The anthers are stout, oblong, and dehisce by an apical pore with a brown line. The ovary is superior, and the stigma is oblong, purple-coloured. Pollination is by insects.
The fruits are flat and stiff pods, oblong-linear or narrow, hairy, and up to 10 cm long.
Unlike many of the other members of this family, this species does not seem to have the symbiotic relationship with soil bacteria with which to fix atmospheric nitrogen.
In the countries where it occurs naturally, the branches of this and related species are often gathered from the wild to make rough brooms and brushes.
The flowers produce much pollen for foraging bees.
From the Yucatan, it is reported that the leaves, when crushed and boiled in water, produce a water that is particularly effective for washing smelly feet!
Photographed in Arcadia 2018
Page last updated 26th November 2018