Remarkable nectaries: structure, ecology, organophyletic perspectives IV. Miscellaneous cases

Abstract

Summary In the present, final part of a series of contributions on nectaries, further, miscellaneous case histories are presented because of their histological or ecological interest. In the special section, extrafloral and floral nectaries of members of six families are treated. A general survey concludes the paper. Extranuptial glands attracting ant guards are associated with the flowers of Becium and Ocimum (Lamiaceae); they represent novel cases in a family hitherto believed to lack this kind of organs. The glands develop in an unusual way by transformation of abscission scars of floral bracts. The five floral but extranuptial glands on the calyx of Siphocampylus betulifolius (Lobeliaceae), with a similar function, are unusual in sharing the same histological structure and drainage via stomatal sap-holes with the closely adjoining nuptial nectary, a disk. Three case histories refer to zoophilous unisexual flowers with only one of the functional sexual morphs rewarding with nectar. In Pachysandra and Sarcococca (monoecious Buxaceae) the male flowers are nectariferous. The nectarless pistillate flowers are inadvertently pollinated by bees while these exploit the staminate flowers. Two other instances, although heterogeneous in detail, exhibit the reverse situation in that only the pistillate flowers yield nectar: the male capitula of the dioecious composite Heterothalamus alienus are girdled by female-sterile ray florets with copious nectar; bees taking it become dusted with pollen when they move over the dry staminate disk florets. They may subsequently pollinate the female heads, which entirely consist of nectariferous pistillate ray florets. In the hummingbird-pollinated Begonia ferruginea it is also the pistillate flowers that contain a presumably nectareous fluid, while the dry staminate ones fallaciously attract the visitors by mimicking the pistillate flowers. Although basically epigynous, the protandrous revolver flowers of the little-known genus Pentaphragma (Pentaphragmataceae)have maintained the subbasal position of their five separate nectaries on the ovary wall, the nectar being accessible to bees via five receptacular channels. In the concluding chapter, possible evolutionary pathways leading to floral nectaries, particularly floral disks, are discussed. Hydathodes of vegetative provenance rather than extrafloral nectaries are likely precursors of most floral nectaries. Developmental modules comprising a single stomatal sap-hole or groups of them, associated with peri stomatal gland cells, may have aggregated in flowers to fonn disks. Separate nectariferous scales as found in flowers of the Cucurbitaceae, Crassulaceae and the Geraniales also reveal similarities with the respective leaf hydathodes. The organophylesis of the single scale seated in the axial spur of Pelargonium is detailed as an example. A clustered distribution of stomatal sap-holes on disks can be partly interpreted as revealing their origin from meristic hydathodal modules. Examples of the diversity of these patterns are given, which are little explored and may provide useful taxonomic characters.