Senna papillosa (Britton & Rose) H. S. Irwin & Barneby
Caesalpiniaceae
Tree: Occasional evergreen treelet (1-5m) notable for its abundant and showy yellow flowers. Associated with secondary vegetation, Senna is found growing in sunny locations – especially alongside roads and trails.
Description: The trunk of this tree is thin, barely reaching 4 cm in diameter even in the largest specimens. Few in number, the branches are long, thin, and flexible, and they often droop under the weight of the foliage they support. Senna leaves (18 cm by 10 cm) are paripinnately compound and alternate. They are composed of four, paired and slightly asymmetrical leaflets of elliptical shape and with tapering, pointed apices. The two distal leaflets are significantly larger (12 cm by 5 cm) than those located proximally, the latter flank a gland located on the petiole between them. In April, an extended period of growth begins during which branches increase in length (sometimes doubling in size) and new leaves are produced.
This growth spurt continues until September or October when terminal panicles of flower buds appear on the branch. Soon, large clusters of cascading, yellow flowers (3-4 cm in diameter) adorn the ends of the limbs. Each blossom is composed of 5 small, brown sepals; five darkly-veined, yellow petals; four short, fat-anthered stamens; and a single long, curved pistil. In form, they strongly resemble most other flowers of the large Senna genus. Blossoming lasts from September through early December.
Fruits are long, bean-shaped pods (30 cm by 1 cm) that can be seen dangling from the tips of the branches. They develop rapidly from the remnants of successfully pollinated flowers, first appearing in November or December. Cylindrical and woody, the pods are organized into hundreds of small, transversely oriented compartments that contain equal numbers of small (2 – 3 mm), angular, and glossy seeds. When the fruit matures, it browns and dehisces longitudinally, allowing the seeds to fall to the ground. Fruiting occurs in February.
Similar Species: Inga punctata has compound leaves of similar shape and arrangement to Senna‘s. In addition to being a much larger tree, Inga also lacks the easy-to-find leaf glands (in this case, small, knob-like structures) located on the Senna petiole.
Natural History: Senna flowers are pollinated by insects, with butterflies notable among them. The dangling and open pods, swaying back and forth at the ends of Senna‘s flexible branches (perhaps pushed by the wind) may spread the seeds over a small area near the parent plant. Since there does not appear to be any other mechanism acting, this seems to be the species’ principle means of seed dispersal!
The Caesalpiniaceae is one of three plant families whose members were once grouped under the general Leguminaceae family name. Nearly all of these plants are legumes – and thus are capable of taking nitrogen from the atmosphere and converting it into usable forms. (Nitrogen, a vital plant nutrient used in protein synthesis, is often lacking in the soil, but it comprises 77% of the air. This atmospheric nitrogen, however, occurs in a triply bonded, molecular form that is impossible for most plants and animals to utilize). By virtue of the presence of special knobs (called nodules) on their roots, legumes maintain colonies of Rhizobium bacteria – organisms that are among the few living things capable of converting molecular nitrogen into the nitrite and nitrate compounds most living things can use. Ultimately, almost all of the nitrogen incorporated into the tissues of plants and animals is made available through this “fixation” process. The legumes – and their associated Rhizobium bacteria – are therefore a special and important group of organisms.
Uses: Due to the beauty of its blossoms, as well as the length and prolific nature of its flowering season, S. papillosa has potential use as an ornamental shrub for gardens, parks, and yards. Nuñez (1978) writes that an infusion made from the fruits of this species has been used as a strong laxative or – in high enough doses – purgative. Most Costa Rican Sennas seem to share this unsettling property.
Related Species: S. reticulata, the renown Saragundi plant, is an interesting species that will not be treated in depth in this guide – though it can be found in a few places in MANP (e.g. around the lagoon between the second and third beaches). This small treelet (rarely over 2m tall) has large, pinnately compound leaves (30 cm) with up to 12 pairs of oval to almost round leaflets. It, too, has yellow flowers but its seeds are borne in flat (not cylindrical) pods. Saragundi merits special note thanks to the lovely nature of its flowers as well as to the medicinal compounds harbored by its leaves. Anti-fungal agents, released by rubbing the foliage on the skin, are reportedly able to cure infections of this type.
Distribution: In Manuel Antonio National Park (MANP), S. papillosa is found alongside trails and roads where light is abundant. Common in Costa Rica, it is native to the lowlands of both slopes. It ranges from southern Mexico to Colombia and Venezuela.
Image: Trunk Flower&Leaf Flower Fruit
Trees of Costa Rica's Pacific Slope 