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(some ecological parameters interfere on nectar secretion such as time within
the season, life history, and light requirements of the plants; Petanidou et al.,
2000).
Galetto and Bernardello (2004), in several
Ipomoea
species (Convolvu-
laceae), pointed out that flower length was correlated with nectary size and
the total volume of nectar secreted, suggesting that structural constraints play
a major role in the determination of nectar traits of these species.
Galetto (1995a) demonstrated in Bignoniaceae that the longer the flower,
the more voluminous the nectary and the higher the stomata number, but
these traits were not related to nectar secretion. On the other hand, Petanidou
et al. (2000) showed in Lamiaceae that the number of stomata does not ap-
pear to influence nectar secretion, in agreement with findings by Teuber
et al. (1980), and Davis and Gunning (1991) in Fabaceae species.
In
Campanula
species (Campanulaceae), reproductive attributes are far
more conservative than flower phenology and pollination-related features
(Blionis & Vokou, 2005); the patterns of change of reproductive attributes
indicate, therefore, prevalence of phylogenetic over environmental con-
straints.
Other studies suggest that flower morphology is evolutionarily more
labile and that corolla traits can frequently change (e.g., Cubas et al., 1999;
Harrison et al., 1999) in comparison to changes in nectar features (e.g.,
Asteraceae; Torres & Galetto, 2002; Fabaceae; van Wyk, 1993; Gesneriaceae:
Perret et al., 2001; Solanaceae; Galetto et al., 1998). In some taxa nectar
composition seems to be a more conservative trait than flower morphology.
The same argument was used to explain the absence of convergence in sugar
composition between plants growing in two different South American bio-
geographical regions (Chaco and Patagonia) that share the same animal
visitor guilds (Galetto & Bernardello, 2003).
It has to be kept in mind that flowers are the most complicated parts of
plants, as they are composed of a number of organs that form an ordered pat-
tern (Endress, 2001a). For instance, in families with sympetalous flowers,
the evolutionary flexibility of floral length and shape provides an excellent
means for isolating mechanisms in pollination and they have a similarly
wide spectrum of pollinators (e.g., Acanthaceae, Bignoniaceae, Gentiana-
ceae, Gesneriaceae, Polemoniaceae, Solanaceae; Grant & Grant, 1965;
Vogel, 1991; Endress, 1994). Even though the key function of nectar is to
attract pollinators, new evidence suggests that selection on flower shape and
