Biology Reference
In-Depth Information
Figure 1.
Detail of epidermis of floral nectary of
Helleborus foetidus
(Ranunculaceae)
.
Nectar
secretion is holocrine and involves degeneration of cell contents and rupture of the outer wall
(cw). Degeneration of secreting cells is not synchronous and secretion lasts several days. Cells
that have already secreted nectar appear almost empty while cells not yet involved in secretion
are still intact. c = cuticle; cr = cytoplasmic remnants; sc = secreting cell; Bar = 8 µm.
Pisum sativum
(Fabaceae) (Davis et al., 1988; Razem & Davis, 1999), and
Eccremocarpus scaber
(Bignoniaceae) (Belmonte et al., 1994).
Pre-nectar passes through plasmodesmata from the phloem parenchyma
cells to the nectary parenchyma cells by the so-called symplastic route
(Fig. 2). Alternatively, pre-nectar flows from sieve elements and companion
cells via intercellular spaces and cell walls to the secretory cells, by the so-
called apoplastic route (Fig. 2). Nectar secretion, i.e., the transport of nectar
outside the protoplast of the secretory cells, may occur by two mechanisms
as described by Fahn (2000):
•
Eccrine secretion is molecular transport of individual sugar molecules
across the cell membrane, possibly by a carrier molecule.
