Remarkable nectaries: Structure, ecology, organophyletic perspectives III. Nectar ducts

Abstract

Summary The present, third part of a series of studies on floral and extrafloral nectaries deals with auxiliary structures whose function is to conduct nectar from its source towards a more or less distant site of exposition. Ducts occur especially in plant groups that have a fixed conformation of floral organs and have been unable to dislocate the nectary towards a site more appropriate for pollination, such as spurs and narrow tubular containers. These drainage systems are concealed, preventing access to visitors. In all known cases, they run externally in capillary clefts between floral organs or in tubes 20–70 μm in diam., formed by invaginations of the cutinized but easily wettable epidermal surface with ± tightly connivent margins. The nectar flows along the ducts driven by capillary forces, secretion pressure and, in part, gravity. The Alliaceae, whose papillate septal nectaries principally bear their three primary outlets at the top of the superior ovary, only rarely discharge the fluid directly at these points, a case exemplified by Allium cernuum. In Brodiaea and Nothoscordum, cryptic ducts run along the external carpellary sutures towards the base of the ovary, where the nectar is released. In Triteleia and Dichelostemma, they continue proximally along an ovarial stipe that extends the distance between the source and the site of nectar deposition. An extreme is achieved in Milla, whose stipe is connate with the perigon tube by means of three septa; each of the three resulting separate nectar pipes is fed with nectar by ducts which run along the stipe over a distance of up to 13 cm, with apertures in the pipe's lower end The zygomorphic flowers of Wachendoifia (Haemodoraceae), with a semi-inferior, trilocular ovary, have only two nectariferous ovarial septa. These are sunken in the torus and drained by lateral, retroverse channels which conduct the fluid to a pair of external, spur-like auricles of the perigon segments. The nectar of Silene schafta and allies (Caryophyllaceae) collects at the bottom of an elongate, tubular calyx. Its source is a disk, borne a considerable distance away – as all remaining floral parts – by a long columnar stipe. The liquid flows within 10 cryptic channels along the stipe and is released at its base above the insertion of the calyx. In Capsicum (Solanaceae), nectar produced by an ovarial disk travels through concealed ducts from the inaccessible tubular part of the corolla towards five apertures which lie outside the staminal fascicle on the flat limb. Analogous drainage systems of the related genera Jaltomata and Physalis are illustrated. The spur of the zygomorphic corollas of Linaria and Kickxia (Scrophulariac.) is supplied with nectar by means of a single duct which descends from the disk and runs down along the dorsal wall to the spur's tip where the fluid is released into the main lumen of the spur. The Violaceae use different strategies to overcome the problem of supplying long spurs with the nectar they produce by distant connectival glands. In Corynostylis volubilis an internal furrow of the spur wall probably assumes this function, leading from the nectar scale to the spur's tip 5 cm away.