Biology Reference
In-Depth Information
kinds of animals. The relative shape, size, behaviour, and mouthparts of visitors
compared to the size and shape of the flowers are essential in determining
access of animals to the nectar and their efficiency as pollinators. In addition
to and as a consequence of exposure, nectar concentration may be affected
directly by corolla depth, since the concentration in flowers with long corolla
tubes is lower than in flowers with short or no tubes, from which water
evaporates readily (Corbet, 1978; Plowright, 1987).
As flowers can be regarded as pollination units, their functional structure
is closely related to their pollination mechanisms. If the well-known struc-
tural blossom classes of Faegri and van der Pijl (1979) are taken into account
and are correlated to nectariferous flowers, several possibilities arise regard-
ing how nectar is offered and can be foraged by visitors within the group of
conspicuous and advertising flowers. This system may be equally applied to
whole inflorescences that actually operate as single floral units, e.g., a
capitulum. Generally, nectar exposure is more intense when flowers are
chasmogamous, opening when shedding pollen. There are several degrees of
exposure, here listed from most to least accessible, according to the different
flower classes (Fig. 2):
•
Dish- or bowl-shaped flowers (e.g., in some Apiaceae, Asteraceae, Ranun-
culaceae, Rosaceae)
•
Head- or brush-shaped (e.g., in some Caryophyllaceae, Combretaceae,
Myrtaceae, Proteaceae, Fabaceae-Mimosoideae, Salicaceae)
•
Bell- or funnel-shaped (e.g., in some Alliaceae, Campanulaceae, Gentiana-
ceae, Rubiaceae)
Alternatively, flower shape may ensure that nectar is comparatively hid-
den and more difficult to forage, as in the next flower classes to an
increasing degree:
•
Gullet-shaped flowers (e.g., in some Acanthaceae, Convolvulaceae, La-
miaceae, Scrophulariaceae)
•
Flag-shaped flowers (e.g., in some Asparagaceae, Fabaceae-Papilionoi-
deae, Papaveraceae-Fumarioideae)
•
Tube-shaped flowers (e.g., in some Apocynaceae-Apocynoideae, Caprifo-
liaceae, Polemoniaceae, Solanaceae)
•
Flowers that have to be opened by visitors forcing their way in (e.g., in
some Fabaceae-Papilionoideae, Polygalaceae, Scrophulariaceae)
