Biology Reference
In-Depth Information
The different ways of nectar emergence and presentation may be related
to one or more of the following:
•
Nectary position in the flower
•
Nectary accessibility from outside
•
The path of flower visitors
•
Mouthparts of flower visitors
•
Number of ovules per ovary
6.2
Extrafloral nectaries
Extrafloral nectar, unlike floral nectar, is always presented on the surface of
the nectary (primary presentation), and there are no reports of secondary
presentation. The most common extrafloral nectaries are situated on leaves
and stems, rarely on fruit. Irrespective of position, the nectar secreted is
always derived directly from photosynthesis by the nectary or other contigu-
ous tissues, generally without storage of starch. This means that the nectar is
always produced during the day and in small quantities. Floral nectar can
often be seen as drops by the naked eye, whereas extrafloral nectar, owing to
its reduced volume, is not perceived as drops but as a shiny surface. The nec-
tar generally does not flow and is rarely lost. In at least some species,
feedback occurs—no further nectar is produced if that present is not col-
lected (Cruden et al., 1983 and references therein).
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FATE OF NECTAR AND NECTARIES
Secreted nectar has different fates. It can either
1.
Be consumed by a pollinator
2.
Be consumed by a nectar thief
3.
Drop from the flower
4.
Remain in the nectary or flower if not removed
The nectar investment is sucessful in the first case, but unsuccessful in
cases 2 and 3 where the environment benefits from the nectar. In case 4 there
are different possibilities depending on the programme of the plant. Nectar
theft is more common when the nectary is exposed, although nectar preda-
tion is not unknown in hidden nectaries or when nectar has a secondary
presentation.
