Senna is a genus of ca. 300 species and widely distributed throughout the tropics (mainly in the Americas). It was formerly included in a very broadly circumscribed genus Cassia L. but differs from the latter in having flattened pods and by the absence of bracteoles (Mabberley 2008). See also Randell (2000). Most species of Cassia in cultivation (in Belgium exclusively as non-hardy pot plants, for instance C. corymbosa Lam. or C. didymobotrys Fresen.) belong to Cassia s.str. Several species of Senna (including those recorded in Belgium) are noxious weeds of agricultural land and now have a pantropical distribution.
Senna in fact belongs with Caesalpiniaceae but this family is now better included in Fabaceae (Mabberley 2008).
In addition to the species mentioned below few others have been reported from other European countries. Records of vegetative plants of Senna marylandica (L.) Link (syn.: Cassia marylandica L.) from Norway (see Melseth & al. 2007) possibly belong with Desmanthus. Likewise, Norwegian records of Chamaecrista nictitans (L.) Moench (syn.: Cassia nictitans L.) (Melseth & al. l.c.) almost certainly pertain to Sesbania herbacea. Curiously, both species were claimed from Finland by Lahtonen & Kääntönen (1992). Sure enough, several species of Fabaceae s.l. are rather characteristic soybean immigrants in many parts of western Europe but nearly all are very reluctant to flower and their exact identity often remains uncertain. The two species of Senna recorded in Belgium are rather characteristic in leaf characters only and their identification is straightforward.
- Leaflets (4-) 6, obovate, obtuse at apex === 1. Senna obtusifolia
- Leaflets 8-10, ovate to ovate-lanceolate, acuminate at apex === 2. S. occidentalis
Cullen J. (1995) Cassia. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 4. Cambridge University Press, Cambridge: 474-476.
Irwin H.S. & Barneby R. (1982) Review of Cassiinae in the New World. Mem. New York Bot. Gard. 35: 1-918.
Lahtonen T. & Kääntönen M. (1992) Naantalin sataman tulokaskasveista II. Mem. Soc. Fauna Flora Fennica 68: 47-62.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Marazzi B., Endress P.K., de Queiroz L.P. & Conti E. (2006) Phylogenetic relationships within Senna (Leguminosae, Cassiinae) based on three chloroplast DNA regions: patterns in the evolution of floral symmetry and extrafloral nectaries. Amer. J. Bot. 93: 288-303.
Mazer S.J. & Dawson K.A. (2001) Phylogenetic relationships in the Caesalpinioideae (Leguminosae) as inferred from chloroplast trnL intron sequences. Syst. Bot. 26: 487-514.
Melseth T.H., Grøstad T. & Halvorsen R. (2007) Blant Ipomoea’er på Place d’Amerique. Om et eventyr blant soyaadventiver sommeren og høsten 1999 – og en delvis nekrolog. Blyttia 65(4): 224-234. [available online at: http://nhm2.uio.no/botanisk/nbf/blyttia/blyttia_pdf/Blyttia_200704_skjermkvalitet_hele.pdf]
Randell B.R. (2000) Cassia and Senna: a review of the groups of plants formerly known as Cassia. Austral. Pl. 20(162): 238-249.