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Taxonomic notes and novelties in the genus Isoglossa (Acanthaceae) from east Africa

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Summary

A review of the genus Isoglossa Oerst. in east Africa is presented in preparation for the FTEA Acanthaceae treatment. Variation in pollen type within the African Isoglossa is summarised and its implications for generic circumscription are considered. The occurrence of periodic gregarious monocarpy (a plietesial life cycle) within Isoglossa is reviewed, with reference to other occurrences of this phenomenon in the Acanthaceae. The relationship of African taxa of Isoglossa to the Asian I. dichotoma (Hassk.) B. Hansen is reviewed and a tabular synopsis and key are presented for this group in east Africa, together with the description of three new species (I. paucinervis, I. ventricosa, I. bondwaensis), one new subspecies (I. membranacea subsp. septentrionalis) and one new name (I. bruceae) within the group. A further three new east African species (I. variegata, I. faulknerae, I. multinervis) and one new subspecies (I. substrobilina subsp. tenuispicata) are described, and the status of I. salviiflora in relation to I. floribunda is reassessed. The conservation status of each taxon is considered.

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Notes

  1. Several taxa have both glandular and eglandular inflorescence forms and so key out in both halves of this couplet.

  2. Populations of Isoglossa somalensis Lindau from the Bale Mts of Ethiopia often have markedly small leaves, a densely glandular inflorescence indumentum and the lateral branching of the inflorescence tends to be suppressed, the cymule clusters becoming subsessile. Elsewhere, the leaves are usually larger and the inflorescence is more widely branched and with a shortly eglandular indumentum only. Intermediate populations are, however, recorded.

  3. Plants of Isoglossa lactea from the northern Eastern Arc Mts (Teita Hills of Kenya and W Usambara Mts of Tanzania) differ from those of the remaining Eastern Arc (E Usambara to Udzungwa Mts) in the corolla tube being strongly swollen and bulbous rather than gradually campanulate or ventricose on the floor. The corolla lobes of this form are also shorter and more rounded, and the flower is larger overall. However, plants from mountains east of the Kenyan Rift Valley and in the Pare Mts of Tanzania are somewhat intermediate.

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Acknowledgements

I am very grateful to Kaj Vollesen and Quentin Luke for their helpful advice and discussion on the genus and on the manuscript, and to Roy Vickery and Vicki Papworth (BM), Dominique Champluvier and Elmar Robbrecht (BR), Frank Mbago (DSM), James Kalema (MHU) and Jim Solomon (MO) for providing access to herbarium collections at their respective institutions. I thank Hannah Banks for providing the SEM pollen images, Andrew Brown and Juliet Williamson for producing the excellent illustrations, Melanie Wilmot-Dear for the Latin diagnoses and two anonymous reviewers for comments on an earlier draft of the manuscript. This research received support from the synthesys Project http://www.synthesys.info/ which is financed by the European Community Research Infrastructure Action under the FP6 “Structuring the European Research Area Programme”.

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  1. Herbarium, Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3AB, U.K.

    Iain Darbyshire

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Darbyshire, I. Taxonomic notes and novelties in the genus Isoglossa (Acanthaceae) from east Africa. Kew Bull 64, 401–427 (2009). https://doi.org/10.1007/s12225-009-9123-5

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