Biology Reference
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a circumstance reported long ago (e.g., Pearson, 1909; Porsch, 1910; van der
Pijl, 1953). Several authors have considered this exudate to be nectar (e.g.,
Jaeger, 1957; Martens, 1971; Singh, 1978; Endress, 1996), but the producing
structure has been named a nectary only by Jaeger (1957). Nevertheless,
Schmid (1988, p. 208) pointed out that this term is improper, as the pollina-
tion drop of gymnosperms and the nectar of angiosperms are not homologous.
He indicated that the use of the terms nectar and nectary for gymnosperms
“applies only in a loose, ecological sense of the words since the primary role
of the pollination drop relates to manipulation of pollen rather than reward to
insects”. In my opinion, the droplets in Gnetales may be regarded as a nectar
reward for pollinators produced by ovular nectaries, according to the broad
topographical classification of nectaries of Schmid (1988). The fact that
these micropylar secretions primarily serve to draw pollen into the inner re-
gions of the ovule where pollen germination takes place, as happens in most
gymnosperms (e.g., Gelbart & von Aderkas, 2002), does not prevent insects
that visit the plants being attracted by the exudate and pollinating them, as
proved in several instances. As this micropylar nectar secretion can take
place either nocturnally or diurnally, the type of insect visitors varies accord-
ingly: moths, flies, bees, and wasps are the most relevant groups recorded
(Meeuse et al., 1990; Kato et al., 1995; Wetschnig & Depisch, 1999). More
importantly, the droplet of these species has a higher sugar concentration
(10-80%; Bino et al., 1984b; Kato et al., 1995; Wetschnig & Depisch, 1999)
than the droplets of the strictly anemophilous conifers (~1%; McWilliam,
1958), and thus, can be considered equivalent to angiosperm nectar.
From the anatomical point of view, the ovular nectaries of
Ephedra
,
Gnetum
, and
Welwitschia
can be regarded as non-structural nectaries. Effec-
tively, they are not histologically or cytologically recognizable (cf. Martens,
1971; Fahn, 1979), but they regularly secrete a solution that contains sugars
serving as reward for pollinators. The secretion takes place at the nucellar
apex (Martens, 1971; Martens & Waterkeyn, 1974; Moussel, 1980; Carafa
et al., 1992). The inner epidermis of the micropylar tube and the integument
may take part in the secretion as well, but this fact is not clear (Martens,
1971; Endress, 1996).
In addition to these ovular nectaries, in some
Ephedra
species nectaries
have been found on the bracts of male and female plants enveloping the re-
productive units and on the integuments of the female plants (Bino & Meeuse,
1981; Bino et al., 1984a, b). In a
Gnetum
species (Kato et al., 1995), in
which sterile female flowers are lacking in male inflorescences, nectar is
secreted between and on the collars. In these taxa both types of nectaries
