Biology Reference
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large multicellular heads, often curved “penitent hairs”) that have been help-
ful in the delimitation of taxa (Jeffrey, 1978).
Some authors (Smets, 1986; Vogel, 1997) consider hypanthial nectaries
to be ancestral in this family. Nevertheless, recent molecular phylogenies
suggest that subfamily Zanonioideae is basal (Jobst et al., 1998), and as
members of its sole tribe (Zanonieae) have sparsely distributed nectariferous
trichomes with small heads on the petals (Vogel, 1997), this type of nectary
can be regarded as primitive. Vogel (1997) regarded the nectaries of Zanoni-
oideae to be anatomically homologous to hydathode hairs, and therefore
primitive compared to the trichomes found in other taxa, which show en-
largement of glandular heads and concentration in dense carpets.
3.5.4
Euphorbiaceae
The members of this cosmopolitan family show a great variety of flower
and inflorescence structures and sexual systems; they are mainly insect-
pollinated and have both floral and extrafloral nectaries that are usually mor-
phologically different and have a distinct evolutionary origin (Webster,
1994). Floral nectaries are extrastaminal or intrastaminal receptacular nectar-
ies, either continuous or five-segmented. Extrafloral nectaries occur mainly
on the leaves.
Several taxa seem to be wind-pollinated; among them are the dioecious
Mercurialis annua
,
in which there is some uncertainty about the existence of
reduced nectaries in the female flower (Daumann, 1972), and the mon-
oecious
Ricinus communis
, which has extrafloral nectaries on the leaf petiole
producing nectar for ants that discourage predators (Nichol & Hall, 1988).
In
Croton
species, both anemophily and entomophily have been reported
(Bullock, 1994; Freitas et al., 2001) and nectaries show great variety. For
instance, the monoecious
Croton sarcopetalus
, has three types of nectaries
(Freitas et al., 2001):
•
Extrafloral (located
in the leaves as two typical glands near the petiole in-
sertion)
•
Floral (five receptacular nectaries in male flowers and ten glands in two
whorls, inner and outer, in female flowers)
•
Post-floral (the outer whorl of nectaries in female flowers continues to se-
crete nectar during fruit development).
In the mainly monoecious
Euphorbia
and related genera, male and female
flowers are very much reduced and are grouped into bisexual pseudanthial
