Biology Reference
In-Depth Information
that these structures have been important in the evolution of certain plant
groups, such as in
Aquilegia
(Ranunculaceae), where the nectar spur represents
a key innovation, i.e., a novel adaptation that allowed for diversification
(Hodges & Arnold, 1994). In other taxa, however, nectar spur morphology
exhibits considerable variation both within and among populations (Herrera,
1988; Travers et al., 2003).
Nectar spurs undoubtedly had independent origins, a hypothesis supported
both by large phylogenetic distances between groups with nectariferous
spurs, and by the different developmental origins of spurs (Hodges, 1997;
APG II, 2003). They should consequently be regarded as convergences.
As they generally develop from sepals or petals, it seems logical to regard
them as sepal or petal nectaries, after the floral whorl from which they were
derived.
There are, however, some genera—e.g.,
Delphinium
,
Aconitum
(Ranun-
culaceae), and
Lobelia
(Campanulaceae)—in which nectar spurs are more
complex structures involving both perianth whorls and associated portions of
the androecium and gynoecium. In these cases, it seems practical just to
name them as nectariferous spurs, and mentioning the floral whorl (or
whorls) from which they originated.
A few plant families are characterized by the universal presence of
nectariferous spurs (Cronquist, 1981): mostly the Asian and African
Balsaminaceae (Travers et al., 2003) and the Central and South American
Tropaeolaceae (Fahn, 1979; Fabbri & Valla, 1998)—both families with sepal
spurs—and the cosmopolitan insectivorous Lentibulariaceae (Narayana &
Satyavathi, 1988; Vogel, 1997), with petal spurs. Some authors regard
the spur in Tropaeolaceae as receptacular (either axial or hypanthial; cf.
Decraene & Smets, 2001), but the general consensus is that it is calycinal.
In the order Ranunculales, there are several families that have some rep-
resentatives with spurs:
•
Some Papaveraceae—specifically the members of the former Fumaria-
ceae—possess petal spurs (Olesen, 1996).
•
In Ranunculaceae,
Aquilegia
has petal spurs, whereas
Delphinium
and
Aconitum
have petal-and-sepal complex spurs (Hodges, 1997; Erbar et al.,
1999).
•
In Campanulaceae, this kind of complex spur also occurs in some Mexican
Lobelia
species (Koopman & Ayers, 2005).
