Canary Island foxglove
Isoplexis is a small genus found on the Canary Islands and Madeira. These two volcanic archipelagos, off the north-west coast of Africa, are part of a larger collection of islands called Macaronesia. Among the many historical visitors attracted to the Canary Islands by climate, biological diversity and economics was the Prussian naturalist Alexander von Humboldt (1769-1859). When he was thirty years old, during a brief visit to Tenerife, Humboldt climbed Teide (3,718 m above sea level). This journey contributed to the ideas he developed about the relationships between plant communities and altitude. Biologists interested in evolutionary biology and adaptation are still attracted to Macaronesia. Among the many lures are those genera, such as Isoplexis, that are unique to Macaronesia.
The novelty of Macaronesian plants has contributed to their attraction in European horticulture since the end of the sixteenth century. The Canary Island foxglove has been part of elite botanical collections in England since the late-seventeenth century. Isoplexis canariensis is commonly found in a narrow, altitudinal band of woodland and laurel forest margins on the islands of La Palma, La Gomera and Tenerife.
Long spikes of bilaterally symmetrical, orange flowers, together with elongate, dark green, leathery leaves and a shrubby habit makes Isoplexis canariensis highly distinctive. Superficially, the plant looks like a woody foxglove (Digitalis). However, its distinctiveness led the Scottish botanist and garden designer John Loudon (1783-1843) to separate it from Digitalis in the early-nineteenth century.
The DNA-based evolutionary tree of Isoplexis shows its closest relatives are all species of Digitalis. This supports the view that Isoplexis is part of Digitalis, as originally proposed by the eighteenth-century Swedish botanist Carolus Linnaeus. Its correct name is therefore Digitalis canariensis. Moreover, Isoplexis is an example of a woody plant that has evolved on the Canary Islands from herbaceous ancestors on the continent.
The robust, orange flowers, arranged in a dense spike, would indicate that Isoplexis canariensis is bird pollinated. However, on the Canary Islands there are no birds that are specialist flower visitors. This led researchers to suggest that the bird pollinators with which the species may have evolved are now extinct. Experimental data have shown that sporadic pollination by non-specialist, flower-visiting birds, such as the Canarian chiffchaff, Sardinian warbler and Canary, is enough to maintain seed production and population survival in the species. Re-examination of the species' biology, e.g., extended flowering time and nectar composition, supports the role of generalist pollinators.
Further reading
Bräuchler, C et al. 2004. Molecular phylogeny of the genera Digitalis L. and Isoplexis (Lindley) Loudon (Veronicaceae) based on ITS- and trnL-F sequences. Plant Systematics and Evolution 248: 111-128.
Dupont, YL et al. 2004. Evolutionary changes in nectar sugar composition associated with switches between bird and insect pollination: the Canarian bird-flower element revisited. Functional Ecology 18: 670-676.
Rodríguez-Rodríguez, MC and Valido, A 2008. Opportunistic nectar-feeding birds are effective pollinators of bird-flowers from Canary Islands: experimental evidence from Isoplexis canariensis (Scrophulariaceae). American Journal of Botany 95: 1408-1415.
Stephen Harris