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Figure 11. The floral nectary of Daphne sericea (Thymelaeaceae). Xylem vessels (xy) stop in
the subnectary parenchyma (snp) while phloem strands branch into the base of the nectary
parenchyma (np). s = stoma. Bar = 80 µm, stained with PAS.
often encountered in nectaries of Asteraceae (Wist & Davis, 2006). Phloem
alone supplies the floral nectaries of most species of Brassicaceae and a direct
relation has been demonstrated between the abundance of phloem supply
and nectar carbohydrate production (Davis et al., 1998).
Well-developed wall ingrowths, reminiscent of those of transfer cells
(Pate & Gunning, 1972), have been detected in the companion cells and are
common in nectary phloem (Davis et al., 1988; Belmonte et al., 1994;
Razem & Davis, 1999; Wist & Davis, 2006). The increased surface area of
the companion cell membrane around these ingrowths is thought to enhance
unloading of pre-nectar components from sieve tube elements and their di-
rect transfer to adjacent phloem parenchyma and intercellular spaces (Davis
et al., 1988; Razem & Davis, 1999; Wist & Davis, 2006). Unusually large
companion cells, characterized by large membrane-bound protein bodies,
were reported in the floral and extrafloral nectaries of different species of
Passiflora (Durkee et al., 1981; Durkee, 1982). In these “intermediary cells”,
wall ingrowths are not evident, but the unloading process may be favoured
by their large surface area. The function of the membrane-bound protein
bodies is obscure.
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