Biology Reference
In-Depth Information
2.3
Subnectary parenchyma
The subnectary parenchyma is located below the nectary parenchyma, from
which it is generally easily distinguished because it consists of larger cells,
with bigger vacuoles, less dense cytoplasm, and larger intercellular spaces.
Durkee (1982) reported plasmodesmata between nectary and subnectary pa-
renchyma cells in the extrafloral nectary of
Passiflora
, suggesting that these
tissues cooperate in the secretion of nectar. Generally neither, the ER nor
Golgi apparatuses in the subnectary parenchyma cells show the unusual de-
gree of development and swelling found in the nectary parenchyma cells
(Durkee, 1983). Insignificant ultrastructural changes take place in subnectary
parenchyma cells approaching secretion, and generally the vacuole increases
in size at secretion.
As described earlier, subnectary parenchyma is generally richer in chlo-
roplasts than nectary parenchyma. Vascular bundles are always present in
subnectary parenchyma. In most cases the xylem vessels stop in this tissue
while phloem strands branch into the nectary parenchyma (Fig. 11).
Oil and mucilage cells were described by Sawidis (1998) in the subnec-
tary parenchyma of
Hibiscus rosa-sinensis
. Because of the water-binding
capacity of mucilage, with rapid water uptake and slow release, it was hy-
pothesized that in this species mucilage cells offer an ideal regulation
mechanism for water balance during nectar secretion and efficient protection
of nectary tissue against water stress damage. Oil cells, on the other hand,
are supposed to be involved in nectary protection against herbivores.
2.4
Nectary vasculature
The vasculature brings raw materials for nectar production to the nectary.
Frey-Wyssling and Agthe (1950) suggested a correlation between the vascu-
lar supply of the nectary and the concentration of nectar. Nectaries that
secrete very concentrated nectar are vascularized by phloem only. Nectaries
secreting nectar with low sugar concentrations are vascularized equally by
phloem and xylem or primarily by xylem. This hypothesis was supported by
observations in
Gossypium
(Wergin et al., 1975),
Abutilon
(Gunning &
Hughes, 1976), and
Hibiscus
(Sawidis et al., 1987a), but was not always
confirmed in subsequent studies (Dafni et al., 1988; Zer & Fahn, 1992).
Although some nectaries are reported to be vascularized by xylem and
phloem, the last branches reaching the nectary parenchyma are generally
phloem elements, which may reach the area of the epidermis. This feature is
