Biology Reference
In-Depth Information
florets are usually nectariferous (Mani & Saravanan, 1999), but in some cases,
only female florets secrete nectar (Vogel, 1998c).
Nectar is secreted through stomata, whose number can vary among spe-
cies (Mani & Saravanan, 1999). The type of vascularization of the nectary is
variable as well—supplied by both phloem and xylem, only phloem or, more
commonly, lacking a special vascular tissue (cf. Wist & Davis, 2006). Sev-
eral types of vascularization can exist in different species of the same genus
(Frei, 1955; Gulyás & Pesti, 1966). In spite of the remarkable practical sig-
nificance of the Asteraceae, nectary anatomy has rarely been analysed in
detail (structure: Frey-Wyssling, 1955; Galetto, 1995c; Torres, 1998; Vogel,
1998c; Sancho & Otegui, 2000; Ma et al., 2002; Visintín & Bernardello,
2005; ultrastructure: Tacina, 1974; Sammataro et al., 1985; Wist & Davis,
2006). More data on the subject with a reasonable taxon sampling may bring
additional revealing results.
3.5.2
Brassicaceae
Four types of receptacular nectaries are distinguished in this chiefly entomo-
philous family, taking into account the location and degree of isolation of the
nectariferous tissue (e.g., Norris, 1941; Deng & Hu, 1995; Davis et al., 1986,
1996, 1998, 1999):
•
Annular type (a continuous nectariferous zone)
•
Four-nectary type (two pairs of glands: the lateral nectaries associated with
the short stamens and the median nectaries external to the long stamens)
•
Two-nectary type (only the two lateral nectaries)
•
Eight-nectary type (two pairs of lateral and two pairs of median nectaries)
In this family, the use of the terms intrastaminal or extrastaminal is not
suitable. Effectively, according to the tetradynamous androecium of the
Brassicaceae, in a species with four nectaries the median nectaries are op-
posed to the two outer shorter stamens and located between them and the
ovary, whereas the two lateral nectaries are alternipetalous, located between
the sepals and the stamens. The lateral sepals are often saccate and serve as
reservoirs for accumulating nectar in some species (Davis et al., 1998).
Davis et al. (1996) demonstrated that in
Brassica rapa
, a species of the four-
nectary type, a percentage of flowers had nectarial connections among the
four glands; the nectary would therefore be better described as an annular
type. In addition, these authors reported variability in several nectary fea-
tures according to the ploidy level in two
Brassica
species (Davis et al.,
1996). In another investigation of members of the family (Davis et al.,
