Biology Reference
In-Depth Information
any aerial plant part could act as a nectar-secreting tissue. As the origin of
the secreted nectar is the phloem sap (Fahn, 1979, 1988), the pre-nectar can
move from the sieve elements to the cells of the neighbouring tissue (either
specialized nectariferous or ordinary parenchymatous tissue) and then to the
plant surface.
Because of their nature, non-structural nectaries are hard to distinguish if
the actual exudate is not observed. The main issue is that when visitors ac-
tively patrol and frequently forage the nectar, no droplets or nectar accumu-
lation can be seen; therefore, the secretion is overlooked. This may be the
reason why non-structural nectaries are apparently rare in angiosperms. So far,
they have been reported in a few families, such as Bromeliaceae (Galetto &
Bernardello, 1992), Cactaceae (Elias, 1983), Costaceae (Zimmermann, 1932;
Elias, 1983), Fabaceae (Vogel, 1997), Melastomataceae (Stein & Tobe, 1989;
Vogel, 1997), Orchidaceae (Frey-Wyssling & Häusermann, 1960; Galetto
et al., 1997), Paeoniaceae (Frey-Wyssling & Häusermann, 1960; Hiepko,
1966), and Ranunculaceae (Kartashova, 1965). Non-structural nectaries may
indeed be overlooked and under-reported among the vascular plants, and
warrant careful observation.
When the nectary is structural, its three main histological components
(epidermis, specialized parenchyma, and vascular system; see Nepi, 2007,
Chapter 3 in this volume) for structural and ultrastructural features) may dif-
fer and, in addition to their functional impact, this variation may be of
systematic value. Let us consider these aspects separately:
Epidermis.
Except for the septal nectaries, which are deeply sunken in inner
floral tissues (Daumann, 1970; van Heel, 1988; Rudall, 2002), floral nectar-
ies are related to the surface of the different floral whorls. The epidermis
may be standard, wholly secretory, or include some secretory structures that
are responsible for producing nectar, which can be referred to as
epidermal
nectaries
in general.
Epidermal nectaries.
Nectar-secreting structures located in the epidermis
are relatively common in flowering plants. Although they can consist of
trichomes, papillae, idioblasts, or glands, they have a common origin.
In 1977, Vogel recognized three basic nectary types: mesenchymatous
(composed of glandular and storage tissues), epithelial
(basically a glandular
epidermis), and trichomatous (secreting trichomes). Recently, Vogel (1998a)
described
nectarioles
as small, few-celled nectaries that are glandular mod-
ules or idioblasts, which can occur singly or in clusters and are anatomically
