Biology Reference
In-Depth Information
Let us now consider how nectaries release their exudate, a feature also re-
lated to nectar presentation. Commonly, nectar is released where it is
produced, called primary presentation by Pacini et al. (2003), i.e., nectar is
exposed to pollinators on the surface of capitate nectaries or on continuous
surfaces. Sometimes, however, the nectar is conducted to other floral parts
and is then termed secondary presentation (Pacini et al., 2003). This is usu-
ally accomplished through nectar ducts—auxiliary structures whose function
is to conduct nectar from its production source to a comparatively distant site
of presentation (cf. Vogel, 1998c). Generally, nectar flows along these ducts
by capillarity, secretion pressure, or gravity. The variability of nectar ducts
within a certain plant group may also be systematically important. Some ex-
amples of secondary nectar presentation are:
•
Several Alliaceae and Xanthorrhoeaceae, whose papillate septal nectaries
principally bear their three primary outlets at the top of the superior ovary,
but only rarely exude nectar there; they have several possibilities, such as
ducts along the external carpellary sutures towards the base of the ovary,
towards the base of the style, or along an ovarial stipe (Daumann, 1970;
Vogel, 1998b; Nepi et al., 2006).
•
In some Haemodoraceae with half-inferior ovaries, septal nectaries are
drained by lateral channels conducting the nectar to spur-like auricles of
the tepals (Simpson, 1993; Vogel, 1998b).
•
In some Caryophyllaceae, nectar collects at the bottom of a tubular calyx
and flows from the disc through ten channels along a columnar stipe
(Zandonella, 1977; Vogel, 1998b).
•
In some Solanaceae (
Capsicum
), nectar is produced by a typical ovarian
nectary from where it flows upwards through ducts from the corolla tube
towards apertures located in the staminal fascicle (Huber, 1980; Vogel,
1998b).
The nectariferous spurs of some Scrophulariaceae and Violaceae use dif-
ferent strategies to supply spurs with the nectar produced by relatively
distant nectaries (Vogel, 1998b; Nepi et al., 2003). However, in other nectar-
iferous spurs, nectar may be produced and accumulated directly in the spurs.
3.1.2
Structure
Nectaries can be divided into two groupings:
structural
and
non-structural
nectaries. Structural nectaries are sharply localized, histologically differenti-
ated areas where nectar is regularly exuded, whereas non-structural nectaries
are non-differentiated areas that are able to secrete nectar sporadically
(Zimmermann, 1932; Daumann, 1970; Fahn, 1979). From this perspective,
