Biology Reference
In-Depth Information
Figure 4.
Nectar exudation in
Cyclanthera pedata.
Nectar is secreted asynchronously by mul-
ticellular capitate trichomes and exuded through ruptured cuticles. Bar = 180 µm.
Spurs may be directed upwards as in Ranunculaceae, or downwards as in
L. vulgaris
(Scrophulariaceae) and many orchids.
The “cuculli” of
Asclepias
(Kevan et al., 1989) are another type of nectar
reservoir—here the nectar flows from the nectary by a capillary system.
Vogel (1998) describes auxiliary structures, named nectar ducts, the function
of which is to conduct nectar from the source towards the site of presenta-
tion, as in septal nectaries (Fig. 3). Nectar flows along these ducts driven by
capillary forces, secretion pressure, and gravity, depending on the orientation
of the nectary and the organs bearing it.
The presence of spurs may imply one or more of the following:
•
Protection against evaporation
•
Consumption by a limited number of animals with long sucking mouth-
parts that can reach into the spurs
•
Protection against contamination by fungal spores and bacteria
•
Long exposure (several days), increasing the chances of the flower being
visited, especially when pollinators are few as is the case of many orchids
(Neiland & Wilcock, 1995)
These hypotheses hold not only for nectar stored in spurs, but also when
nectar is hidden in corolla tubes or septal nectaries. Nectar presentation may
be a species-specific characteristic and may vary even in allied species or
members of the same family. Different types of nectar presentation are asso-
ciated with certain advantages and disavantages (Table 5).
