Remarkable nectaries: structure, ecology, organophyletic perspectives I. Substitutive nectaries

Abstract

In the present, first part of a series of studies on floral and extrafloral nectaries (N, Ns) that deserve interest from the structural, taxonomic, evolutionary and ecological points of view, the nuptial Ns found in members of five families are examined. They are considered to be substitutive, having evolved secondarily in subgroups that have lost another, non-homologous type of N common in their intrafamilial affinity, or that occur within an alliance that lacks floral Ns, but is presumably derived from an ancestry bearing a non-homologous type of N. The subtribe Genistinae (Fabaceae) is characterized by a closed filament tube and suppression of the nectariferous intrastaminal disk. In consequence, typically only pollen serves as a reward for pollinators. Chamaecytisus, Petteria, and Retama, to which Erinacea and Spartocytisus are added, are known to be exceptions. Field data, including behaviour of pollinators (mainly bumblebee queens), prove that they possess true nectar flowers. The site of nectar production was unknown. In the two cases examined, the liquid was found to flow out of two lateral external non-stomatic pore fields of the filament tube. As no glandular tissue is developed, the nectar probably represents phloem sap from leaking adjacent filament bundles. The taxa involved apparently regained nectar production in this distinctive manner in partly independent lineages. The perigynous disk of non-related Stylosanthes (Fabaceae) is obliterated owing to extreme elongation of the receptacle, which forms an empty “exposition tube”. Compensating this deficiency, apparently non-homologous nectar glands are seated at the tube's distal end in several species, accessible for short-tongued bees. Other species of the genus and species of related Arachis (with receptacular tubes up to 8 cm long) have closed filament tubes and no nectar. Complementing the author's previous results, atypical Ns occurring in the Primulaceae are described. In apetalous Glaux, single glandular trichomes scattered over the ovary produce the nectar, conforming to the tribe Lysimachieae, which displays (besides elaoiphores) only trichomatous Ns derived from hydathodes. Dionysia and Coris, on the other hand, share the mesenchymatous carpellary gland type of tribe Primuleae, a presumably more recent acquisition. Contrary to former belief, nectariferous Myrsinaceae do exist (Maesa). Further examples of trichomatous Ns, replacing the stomatal mesenchymatous Ns predominating in the Cucurbitaceae, are shown in species of Sicyos, Microsechium and Sechium (tribe Sicyeae), Echinocystis, Elateriopsis (Cyclanthereae), and six genera of the subfamily Zanonioideae. Trichomatous Ns, anatomically homologous to hydathode hairs, prove to be a constant feature of the three clades. While retaining a primitive level in the zanonioid members, Ns of the two other taxa exhibit parallel trends towards enlargement of glandular heads, concentration to dense carpets and internalization leading to euphily of the flowers. Concealment reaches its extreme in sect. Frantzia of Sechium, with nectariferous carpets enclosed inside ten chambers of the inflated torus. Although probably rooting in an ancestry bearing receptacular Ns, the Melastomataceae typically possess dry pollen flowers. However, as known from Neotropical members of five tribes, a sugary solution may be discharged through spontaneous ruptures of the filaments. Since nectarial tissue is absent, the fluid stems from the leaking filament bundle rich in phloem. This unique manner of nectar production was apparently archieved in independent, parallel progression lines, based on a common propensity. In addition to previous reports, novel cases are examined in species of Brachyotum, Tibouchina, Centronia and Miconia. Nectariferous fissures in most cases develop ventrally, loosely connected with the filament's prior curvature (geniculum) in the bud stage, but are independent and dorsal in Centronia. Nectaring of the petal tips in the flower bud of Medinilla magnifica, extranuptial in function, is unrelated to the foregoing situation. In this case, it rests on stomatal Ns with a pattern homologous to hydathodes seated on the margin of the foliage leaves. Some species of Sambucus, a genus basically devoid of nuptial Ns, have secondarily achieved this attractant in two different ways. Apart from the better known case of S. ebulus with a nectaring stigma, in S. javanica and relatives, numerous large cuplike nectariferous glands are distributed between the (dry) florets of the corymb, where they are exploited by insects. Hitherto taken for transformed flowers, the glands are evidently homologous to leaf-borne extranuptial Ns that occur in the same species and are widespread in the genus.