INTRODUCTION - Nectaries and Nectar - page 2

The reward(s) to pollinating insects in early angiosperm flower evolution
were floral secretions, and not pollen, in contrast to earlier hypotheses.
The pollination drop on the ovular micropyle and later the stigmatic
secretion may have served as nectar reward for pollinators...
The first two lines of Tahktajan's statement were disputed in the light of
new evidence from different sources, but Endress also presents the concept
that nectaries may have multiple evolutionary origins and can be induced in
a wide range of positions and tissues in the flower (Simpson & Neff, 1983).
The history of ideas about nectaries and nectar shows that it was a long
time before the role of nectar in insect pollination was recognized: earlier it
was assumed that nectaries originated as excretory organs to rid the flower of
superfluous liquid (Lorch, 1978). This physiological explanation for nectar-
secreting structures was recently revived by de la Barrera and Nobel (2004),
in the context of the carbon and water relations of flowers. According to
their “leaky phloem” hypothesis, nectar secretion could result from high
hydrostatic pressure in the phloem and the structural weakness of developing
phloem tissue. Their complementary “sugar excretion” hypothesis is based
on sugar accumulation due to rapid growth and associated high transpiration
rates of floral structures. However, we consider the primary function of nec-
taries to be ecological rather than physiological, as sites where liquid
substances involved in interactions with animals are produced and offered in
exchange for benefits to the plant. Animals that are attracted by nectar re-
wards not only involuntarily disperse pollen in the environment, thus
enabling plants to avoid self-fertilization and competition with parents and
siblings, but may also help protect plants from herbivores.
The most ancient extant plant with nectaries is the bracken fern Pteridium
aquilinum , which has extrafloral nectaries on its fronds (Heads and
Lawton, 1985). The phylogeny of extant seed plants shows three separate
origins of animal pollination: in cycads, gnetaleans, and angiosperms (Pellmyr,
2002). In gymnosperms, secretions resembling nectar occur in Gnetales
and are involved in pollination (Bino et al., 1984; Wetschnig & Depisch,
1999). Nectaries are far more common in angiosperms, dating back to the
late Cretaceous. Early-branching lineages of the angiosperms (the ANITA
grade, based on molecular studies) are characterized by tiny flowers with wet
stigmas, the stigmatic secretions being a potential reward, included among
plesiomorphic traits in angiosperms (Endress, 1994a, 2001). Most angio-
sperms are pollinated by insects, which are rewarded with nectar during
visits to flowers with floral nectaries, whereas extrafloral nectaries reward a
more limited set of animals, mainly ants, that keep herbivores away.