INTRODUCTION - Nectaries and Nectar - page 6

3 FLORAL AND EXTRAFLORAL NECTARIES

Two types of nectaries, floral and extrafloral, were recognized by Bonnier
(1879). They may differ considerably in anatomical structure, source of nec-
tar components, and mode of presentation (Davis et al., 1988; Pacini et al.,
2003). Nevertheless, both have the same function: to reward animals that
provide the mobility which plants lack—vectors for pollen dispersal and ants
for physical defence—and their exudates are chemically similar. Floral nec-
taries, however, are better known than extrafloral ones and receive more
attention in this volume. The reason for this “asymmetrical knowledge” is
that floral structure and the different reproductive strategies of plants have
long intrigued biologists and have resulted in comparatively more attention
being directed to floral nectaries. These nectaries are also important sources
of food for honeybees and are involved in the reproduction of many plants of
economic significance and in the production of many fruit and seed crops.
Extrafloral nectaries, which occur mainly in tropical plants, are noteworthy
from an ecological point of view, but have limited economic applications, at
least at present. Some of the differences between floral and extrafloral nec-
taries are summarized in Table 1. Koptur (1992) provides a detailed review
of the interactions between insects and plants mediated by extrafloral nectar-
ies.

Extrafloral and floral nectaries may be found in the same plant species
with their secretion being collected by different kinds of animals. The struc-
ture, composition, and ecology of extrafloral and floral nectaries in the same
species have been compared in various papers, e.g., Croton sarcopetalus ,
Euphorbiaceae (Freitas et al., 2001); Tabebuia serratifolia , Bignoniaceae
(Thomas & Dave, 1992); Thryptomene calycina , Myrtaceae (Beardsell et al.,
1989); Turnera ulmifolia , Passifloraceae (Elias et al., 1975).

The distinction between floral and extrafloral is topographical, but this

separation is artificial. The distinction is certainly not clear in the genus

Euphorbia , where the extrafloral cyathial nectaries are very close to the flower

and are involved in pollination even if this is not clearly stated in the litera-
ture (Proctor et al., 1996, Fig. 2.16). Floral nectaries of Ruellia radicans
(Acanthaceae) produce dilute nectar collected by hummingbirds. The nectary

does not cease its secretory activity after the corolla has fallen, but conti-
nues producìng nectar with a higher sugar concentration, collected by ants

(Gracie, 1991). This example is important because it demonstrates that nec-

tary cells may produce nectar with different concentrations according to deve-

lopmental stage and ecological necessities. The higher concentration of nectar