Biology Reference
In-Depth Information
inflorescences called cyathia (Cronquist, 1981; Webster, 1994). These struc-
tures have bracts with tips that alternate with nectary glands (Hoppe, 1985);
these are considered extrafloral, but are positioned on an extremely reduced
inflorescence (functionally a flower) and certainly attract pollinators (Reddi
& Reddi, 1985). Pollinators are mainly small generalist insects (Ehrenfeld,
1976), although in
Pedilanthus
the more or less radially symmetrical cy-
athium of
Euphorbia
has been highly modified into a bilateral, spurred
pseudanthium that is pollinated mainly by hummingbirds (Dressler, 1957).
3.5.5
Ranunculaceae
Except for a few wind-pollinated species (in
Thalictrum
), most taxa of this
cosmopolitan family are animal-pollinated (mostly by bees, flies, moths, and
birds) and offer easily accessible nectar or pollen as reward (Vogel, 1993;
Erbar et al., 1999).
Caltha
is exceptional in possessing nectar-secreting
trichomes located on either flank of each carpel (Smets & Cresens, 1988,
who consider these nectaries as gynopleural).
In the remaining nectariferous species, nectar is produced by special nec-
tary organs (previously known as honey leaves or nectary leaves; cf. Schmid,
1988) that are located between the perianth and the stamens (Kosuge, 1994);
as they are mainly considered to be petals (although some authors treat them
as tepals) they are petal nectaries. In some genera (e.g.,
Nigella
,
Helleborus
),
the whorl of nectaries is arranged to form a revolver blossom (perambulatory
apparatus), which intensifies the visitor's anther and stigma contact (Vogel,
1993).
The nectaries in this family are very variable in terms of number, shape
(cup-shaped, flat, spurred, peltate, epeltate), and presence, and they have
been systematically utilized in the delimitation of subfamilies, tribes, genera,
and subgenera (e.g., Tamura, 1966, 1967, 1968; Dahlgren, 1992).
The nectary organs are supposed to have been derived from stamens (cf.
Tamura, 1993). After developmental analyses, Erbar et al. (1999) found a
presumed relationship of nectary organs and stamens, which does not compel-
lingly imply it. These authors believe that nectaries may be phylogenetically
“interpreted as organs which developed during the early evolution of nectar-
offering flowers by an overlap of the genetic programs of the perianth mem-
bers and the following stamens during floral ontogeny”.
