Biology Reference
In-Depth Information
2002; Ickert-Bond & Wojciechowski, 2004) suggests that entomophily, ac-
companied by nectar secretion, may be considered a synapomorphy for this
order and that it has been important in its evolution.
3
NECTARIES IN ANGIOSPERMS
3.1
Diversity
Floral nectaries are comparatively simple structures of different origins that
are involved in the pollination process. They can occur in virtually all parts
of the flower and produce a variety of sugary exudates. Even though for
more than a century the shape, structure, and position of these nectaries has
been used for taxonomic and phylogenetic considerations (e.g., Behrens,
1879; Bonnier, 1879; Knuth, 1906-1909; Brown, 1938, 1961; Norris, 1941;
Fahn, 1953; Kartashova, 1965), only relatively recently has the systematic
value of nectaries been acknowledged (e.g., Daumann, 1970; Zandonella,
1977; Fahn, 1979; Cronquist, 1981, 1988; Vogel, 1981a; Smets, 1986, 1988;
Smets & Cressens, 1988; Smets et al., 2000, 2003). As has happened with
many other reproductive characters, nectaries have, to some extent, been
overlooked by systematists. These specialized structures can, however, show
evolutionary trends in plant groups because they are either reasonably ho-
mogeneous, or because they have dramatically changed within groups. In
addition, they may be easily lost or acquired within a lineage, helping to un-
derstand the evolution of the group and of its mating systems.
In angiosperms, the diversity of nectaries may be related to several
causes involving three basic aspects of nectary biology: nectar presentation,
nectary structure, and nectary fate. As a whole, the extent to which these
variations are correlated with the delimitation and history of coherent plant
groups is largely unexplored. Nevertheless, all these aspects may reveal sig-
nificant trends and should be taken into account when trying to elucidate
them. In addition, symmetry, number, and colour of nectaries are of interest
in evaluating their diversity.
3.1.1
Nectar presentation
Floral nectaries may either be at the same level as the surface of the organ
that bears them, form a protrusion, or be deeply embedded (Fahn, 1979,
1988). In addition, how nectar is presented within the flower—i.e., its degree
of exposure—is highly significant from the functional point of view. Thus,
the more exposed the nectar, the easier it is to reach and remove for different
