Biology Reference
In-Depth Information
In
Menyanthaceae
, which has superior ovaries, an annular ovarian nectary
surrounding the base is present (Cronquist, 1981; Erbar, 1997). In
Penta-
phragmataceae
, nectaries are distinctive: unlike most families with inferior
or semi-inferior ovaries in the order, which normally have epigynous glands,
there are five near-basal nectaries on the ovary wall (in its free part); nectar
is accessible by five tubular receptacular channels located between the sta-
mens that have protecting trichomes at their entrances (Vogel, 1998c). In
Stylidiaceae
(including Donatiaceae), two epigynous nectary glands (usually
unequal in size) are located at the base of the column (Cronquist, 1981; Erbar,
1992), although they may be absent in some taxa (Laurent et al., 1998).
Dipsacales.
Nectaries mostly consist of unicellular trichomes and an under-
lying nectariferous tissue associated with the corolla tube (Wagenitz &
Laing, 1984). In
Caprifoliaceae
s.l.
(including Diervillaceae, Dipsacaceae,
Linnaeaceae, Morinaceae, and Valerianaceae), nectaries are frequently rep-
resented by one flat cushion-like nectary in an abaxial position in the corolla
tube, which may be saccate or spurred; there can also be five glands or a
continuous ring (Brown, 1938; Fahn & Rachmilevitz, 1970; Weberling, 1977;
Wagenitz & Laing, 1984; Davis, 2003). On the other hand,
Adoxaceae
is
variable: there may be (i) nectaries in groups at the base of each corolla lobe,
with multicellular trichomes not accompanied by an underlying nectariferous
tissue (
Adoxa
), (ii) a gynoecial nectary on top of the inferior ovary (
Vibur-
num
), or (iii) no nectaries (
Sambucus
) (Wagenitz & Laing, 1984; Erbar, 1994).
Based on these differences, Wagenitz and Laing (1984) proposed that these
genera constitute a monophyletic group, a suggestion that was confirmed
with molecular data (Bell et al., 2001); the basal nectary type of the order is,
however, hard to determine. For
Sambucus
, there are reports of sterile flow-
ers transformed into nectaries and stigmatic exudate as nectar (Vogel, 1997).
3.9
Evolutionary trends
The evolutionary history of nectaries and their potential for elucidating plant
phylogenies is still an unexplored field of plant research (Vogel, 1997). In
the previous section, the amazing diversity of nectary structures and distribu-
tions became evident, a circumstance that suggests that they have had
multiple independent origins, as Brown (1938) pointed out (p. 549): “Nectar-
ies appear to have arisen independently in different lines of development and
then to have undergone modifications characteristic of various groups”. On
the other hand, nectaries are easy to lose and reacquire within lineages, and
thus it is sometimes difficult to determine the precise evolutionary sequence.
Last but not least, determining nectary homologies is essential to gain a clear
