Biology Reference
In-Depth Information
on live plants are necessary to detect nectaries with accuracy. The remark-
able diversity within most groups, the inadequate number of taxa studied,
and the rare analyses of intraspecific variation available, all contribute to our
insufficient knowledge of the distribution and structure of nectaries within
higher-level taxonomic groups.
In some plant groups, there is a certain level of homogeneity in terms of the
presence (e.g., Brassicaceae, Bromeliaceae, Convolvulaceae, Heliconiaceae,
Lamiaceae) or absence of nectaries (e.g., lineages with abiotic pollination as
in Betulaceae, Casuarinaceae, Ceratophyllaceae, Cyperaceae, Potamoge-
tonaceae, or with other floral rewards as in Actinidiaceae, Calceolariaceae,
Commelinaceae, the former Krameriaceae (now in Zygophyllaceae), some
Papaveraceae). In the many nectar-bearing families, nectary structure and
position may be reasonably comparable among the members (e.g., Asteraceae,
Bignoniaceae, Brassicaceae, Crassulaceae, Rubiaceae), or may vary in mor-
phology and location (e.g., Cucurbitaceae, Euphorbiaceae, Orchidaceae,
Ranunculaceae, Thymelaeaceae). On the other hand, nectaries can be present
or absent in members of the same plant assemblage (e.g., Amaranthaceae,
Cornaceae, Phytolaccaceae, Plantaginaceae, Salicaceae, Solanaceae). As ex-
amples of these dissimilar scenarios, some patterns within a few plant groups
follow to show the extraordinary variation that floral nectaries can achieve.
3.5.1
Asteraceae
This highly successful, wide-ranging family, with more than 22,000 species,
has nectariferous inflorescences (capitula) that also offer pollen as reward. In
spite of its diversity, the family is comparatively homogeneous in the pres-
ence of a floral nectary. These nectaries are small, annular, and located on
top of the inferior ovary surrounding the style base (Brown, 1938; Frei,
1955; Cronquist, 1981; Mani & Saravanan, 1999). It seems more accurate to
consider these nectaries as ovarian rather than stylar (as Fahn, 1979), since
histologically they are definitely related to the ovary. According to the exter-
nal morphology, nectary shape and size are extremely variable (Gopinathan
& Varatharajan, 1982; Mani & Saravanan, 1999) and may have systematic
value. There is also variability within a single capitulum, where some florets
may lack or have vestigial nectaries. For instance, disc hermaphroditic flo-
rets are usually nectariferous, whereas ray female or neuter florets either lack
or have inconspicuous nectaries (Mani & Saravanan, 1999). The situation
becomes even more complex when one considers the different types of capitula,
the kind of sexuality of the species, and the enormous diversity of the family
(Mani & Saravanan, 1999). For instance, in most dioecious taxa only male
