Rare Plant Discovered
in Champaign-Urbana Area

During this fall, I discovered two plants of Cassia obtusifolia (Sicklepod) growing along the railroad tracks in the Champaign-Urbana area. The plant in the photographs was growing in railroad ballast along the Red Bison Prairie in Savoy. The other plant was growing in a waste area along the railroad tracks in north Champaign. Another scientific name for this plant is Senna obtusifolia. Sicklepod is a rare plant in Illinois. According to official records, it has been observed in only a few southern counties of the state and the Chicago area, growing along railroads or along streams. Most of these observations occurred before 1920. To my knowledge, this is the first time Sicklepod has been observed in east central Illinois. It is a more common plant in the southern United States. The common name is derived from the shape of the seedpod, which is long and curved like a sickle. A seedpod can be found in the lower right corner of the photograph below.

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Sicklepod is a member of the Caesalpiniaceae (Caesalpinia family). Other members of this family that grow in the area are Cassia fasciculata (Partridge Pea), Cassia hebecarpa (Wild Senna), and Cassia marilandica (Maryland Senna). Sicklepod resembles these latter two species, except that it is an annual plant and somewhat shorter (about 2 feet tall). It has evenly pinnate compound leaves with a maximum of 6 leaflets; there is an extra-floral nectary, resembling a small brown spike, at the base of the lowest pair of leaflets. These nectaries attract ants, wasps, and other insects. Their purpose is not entirely clear: they may distract insects that are ineffective pollinators away from the flowers, or they may attract insects that provide some measure of protection from foliage-eating insects or mammalian herbivores. The yellow flowers are about 1" across and sparingly produced during late summer and fall. They are pollinated primarily by the larger bees, such as bumblebees. Below is a close-up of the flower and one of the extra-floral nectaries.

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It is unlikely that Sicklepod is native to east central Illinois. The observed plants are probably adventive from the southern United States, and brought into this area by railroad trains. There are at least two ways that this can be accomplished: 1) Railcars with livestock sometimes release manure that contains the viable seeds of various plant species, while the train is rumbling down the tracks, and 2) Railcars loaded with piles of grain often contain the seeds of field weeds and other plants, which are blown away from the train by the wind. It remains to be seen if a small population of this rare plant will become established in this area. Generally, annual plants favor disturbed areas where they don't have to compete directly against deep-rooted perennial plants. Their populations can fluctuate significantly from year-to-year, depending on weather conditions and other factors.