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Substitute provides the sweet taste of success
Appearances can be deceptive. Classic examples are those organs which, on a cursory glance, appear to be petals but are in fact sepals or even bracts. These substitutes are, nevertheless, fulfilling the biological role of petals and thus have, over the course of evolution, acquired the relevant petal-like features. Another example of substitution is provided by the work of the Brazilian team, based at Recife, working in collaboration with the University of Vienna (Lopes et al., pp. 169-174). They study pollination mechanisms in the Bignoniaceae, a large family confined to new world tropics, all members of which are pollinated by animals, ranging, in different species, from bees to lemurs. Many possess, at the base of the ovary, a disc-like nectary from which nectar is secreted via modified stomata. However, several genera lack a nectariferous disc, or if they possess a disc it is non-secretory; some of these simply mimic nectar-providing flowers and thus deceive the pollinator. However, in the genus Lundia, all members of which are pollinated by humming birds, the function of the disc is fulfilled by a substitute, a 'carpet' of multi-cellular trichomes located on the inner surface of the corolla. These secrete their nectar into the base of the corolla tube thus providing a 'typical' feeding site for humming birds. Nectariferous trichomes have in fact been described elsewhere, including, somewhat surprisingly, the stipules of Vicia sepium. However, this paper gives us an interesting and unusual example of a substitute nectary providing nectar within the corolla. The authors surmise, with some justification, that the development of these substitutes occurred in evolution after the prior loss (or at least loss of function) of the nectary disc. Thus the evolutionary ancestors of these Lundia species may have been deceptive mimics. The substitute nectaries have re-instated the pollinators' reward.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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