INTRODUCTION - Nectaries and Nectar - page 7

Table 1. Summary of the main differences between floral and extrafloral nectaries.

Floral nectaries Extrafloral nectaries

Function Reward animals transporting

pollen

Reward animals defending
plant from herbivores

Position In different parts of flower:

ovary, stamen, calyx, corolla,
receptacle

Common in leaves: petiole,
stipule, blade
Less often in developing
inflorescence, e.g., Euphor-
bia , on floral parts (e.g., calyx
or corolla) and developing or
mature fruit, e.g., certain
Bignoniaceae (Thomas &
Dave, 1992)

Nectar consumers Insects: especially Hymenop-

tera, Diptera, Lepidoptera
Birds: e.g., hummingbirds,
sunbirds
Mammals: e.g., bats, small
marsupials

Mainly ants

Duration of secretion Few hours to several days,

rarely exceeding a week as in
Helleborus (Vesprini et al.,
1999)

Few days in “tender” young
growth, few weeks (when in
fruit) to months (nectaries last
as long as leaves)

Amount of nectar produced Less than 1 µl to few ml:

proportional to the nectary
parenchyma volume

Generally few µl per day

Variability of nectar quality Chemical and physical fea-

tures (viscosity) vary widely
in relation to different nectar
consumers

Nectar physicochemical fea-
tures vary less because ants
are main consumers

during the extrafloral phase may be related to greater exposure of these pho-
tosynthesizing nectaries to light once the corolla falls.

Benefits to plants from the associations between ants and extrafloral nec-
taries are not always obvious. Extrafloral nectaries of ferns are best studied
in the cosmopolitan bracken P. aquilinum , and are hypothesized to provide
rewards for ants that defend the plants from herbivores. Experiments with
British populations of bracken have, however, seldom provided any evidence
that ants visiting foliar nectaries influence levels of herbivory (Heads and
Lawton, 1985; Heads, 1986). In South African populations these extrafloral
nectaries confer protection only when ant densities are high and homopterans