Postpollination nectar reabsorption in the African epiphyte Aerangis verdickii (Orchidaceae)

Nectar reabsorption in flowering plants seems to be a rare phenomenon and to our knowledge it has not been reported previously in the Orchidaceae. In this study we present data that show statistically significant differences in nectar sugar concentrations before and after pollination. Virgin flowers of the African epiphytic orchid Aerangis verdickii showed mean sugar concentrations in the nectar of 14.4 and of 18.3% in 1994 and 1995, respectively (sucrose mass by mass equivalent). Sugar concentration in nectar from naturally pollinated flowers had a mean of 3.2 and of 8.7% for the same period. In an experiment in the wild 60 flowers were protected against nectar theft by the ants. Half of those flowers were hand-pollinated. Control (virgin) flowers had nectar with a mean value of 13.8%, while flowers measured 48 h after pollination had a mean sugar concentration of 4.3%. The mean volume of nectar based on measurements of 23 virgin flowers from nine different plants was 19 uL. We estimated an average energy value for the nectar produced of 684 J per plant per season. At the study site, over 60% of the unmanipulated flowers were robbed of all their nectar by arboreal ants, Polyrachis spp. (Formicinae). The adaptive significance of nectar reabsorption for A. verdickii is probably a function of the environmental stresses to which it has been exposed and the relative costs of nectar production.     

Hawk-moth scale analysis and pollination specialization in the epilithic Malagasy endemic Aerangis ellisii (Reichenb. fil.) Schltr. (Orchidaceae)

The pollination biology of the epilithic endemic Aerangis ellisii (Reichenb. fil.) Schltr. (Orchidaceae) was studied on an inselberg in the Antananarivo region, central Madagascar. The flowers exhibit long nectariferous spurs indicative of hawk-moth pollination, the mechanism evidently involving pollinia transfer via the frons and palpi of the vectors. Analysis of hawk-moth scales on naturally pollinated stigmas showed that the principal pollinators were Agrius conuolvuli (Linnaeus) and Panogena lingens (Butler). Proboscis morphology of long-tongued Malagasy Sphingidae in relation to nectar position and spur morphology in A. ellisii also indicated that A. conuolvuli and P. lingens were best suited to interact with the plant's floral adaptation. Aerangis ellisii seems to display a relatively moderate specialization versus the Malagasy hawk-moth guild since its nectar is accessible even to those long-proboscis hawk-moth species which are not able to act as pollinators.     

Plate 447. Didymosalpinx norae

Didymosalpinx norae (Swynn.) Keay ( Rubiaceae ) is a shrub native of eastern Africa, with a disjunct distribution. It occurs in the rain forests of the Indian Ocean coastal belt and in submontane forests of the Chimanimani range at the border of Mozambique and Zimbabwe. Living material at the National Botanic Garden in Belgium is used here to illustrate and describe it. Taxonomic history, distribution, habitat and cultivation requirements are also discussed. The affinity of the genus Didymosalpinx raises some questions. The material used here was also included in recent phylogenetic analyses shortly reviewed here; its belonging to the tribe Gardenieae is questionable.     

Plate 384. Jasminum leptophyllum

Jasminum leptophyllum R.Rafiq (Oleaceae), an endemic of Pakistan, is illustrated and its discovery, taxonomy, habitat, introduction to cultivation and cultural requirements are discussed. Notes on the relationship to other Jasminum species are also provided.