Biology Reference
In-Depth Information
3
GYNOPLEURAL (SEPTAL) NECTARIES
Fahn (1979a) formulated a topographical classification of floral nectaries,
differentiating nine different types. Among them, the “ovarial nectary” type
includes nectaries that are situated in the septal region between adjacent car-
pels, known as septal or, more recently, gynopleural nectaries (Smets &
Cresens, 1988). Gynopleural nectaries are largely absent in dicotyledons,
although there are non-secretory septal slits in
Saruma
(Endress, 1994),
Cneorum tricoccum
,
Koelreuteria paniculata
,
Ruta bracteosa
and a few other
dicotyledons (Schmid, 1985). On the other hand, they are the most common
type of floral nectary in monocotyledons (Smets et al., 2000, Table 1) and
are therefore considered separately from the other types of floral nectaries.
According to Rudall (2002), septal nectaries have been lost several times in
monocot evolution, probably in association with the emergence of different
pollination syndromes.
The gynopleural nectary, being a cavity inside the ovary, is not directly
exposed to nectar-feeding animals and the site of nectar emission is often
different from the site of nectar production (secondary presentation). Nectar
must therefore flow through auxiliary ducts—up to 13 cm long in
Milla
biflora
—to reach its site of emission (Vogel, 1998b; Bernardello, 2007; Pacini
& Nepi, 2007). The morphological characters of gynopleural nectaries were
reviewed from a systematic point of view by Daumann (1970) and Smets
et al. (2000) and from a functional perspective by Schmid (1985). A very thin
and sometimes apparently discontinuous cuticle is present on the surface of
epithelial cells (Fahn & Benouaiche, 1979; Nepi et al., 2005). The nectar
cavity is lined by a layer of secretory epithelial cells that may overlie a sub-
sidiary glandular tissue, characterized by smaller cells with denser cytoplasm
than the ground parenchyma cells, thus resembling the nectary parenchyma
of floral nectaries. Wall ingrowths are very common in epithelial cells that,
for this reason, are regarded as transfer cells. The differentiation of transfer
cells in septal nectaries is supposed to be an anatomical device to increase
nectar output via eccrine secretion (Schmid, 1985). Cell wall ingrowths are
highly developed in
Aloe
and
Gasteria
(Schnepf & Pross, 1976; Nepi et al.,
2005), but are not so abundant in the nectaries of banana and
Tillandsia
(Fahn & Benouaiche, 1979; Cecchi Fiordi & Palandri, 1982), where pre-
dominantly granulocrine secretion seems likely. Different extents of the
subsidiary tissue were observed in different species of
Tillandsia
(Cecchi
Fiordi & Palandri, 1982) and were related to nectar production rates.
The development of septal nectaries follows two patterns that differ
mainly in the fate of the nectary after the secreting phase. A breakdown of
