Biology Reference
In-Depth Information
mainly those investing heavily in nectar production and having many ovules
per ovary, such as
Cucurbita pepo
,
Linaria vulgaris
, and certain orchids
(Pacini et al., 2003).
The stigma of many plant species exudes liquids consisting mainly of lip-
ids that facilitate pollen adhesion, but in some monocots (e.g., certain
Araceae) the stigmatic secretion is a clear sugary fluid containing few or no
lipids (Heslop-Harrison & Shivanna, 1977).
The watery exudate forming
drops on the spadix of
Anthurium hookerianum
contains 7% sugar (Vogel,
1983). In
Asclepias syriaca
the exudate produced by the stigmatic chamber
moves through a capillary system to nectar reservoirs, which are visited by
insects. The nectar thus functions as both germination medium for pollen
and reward for pollinators. Its concentration may increase through evapora-
tion, but rates of pollen germination are highest in sucrose solutions of
11-15% w/w (Kevan et al., 1989). This shows a clear link between the
funtions of stigmatic secretions and nectar.
In the vegetative and reproductive organs of angiosperms there are other
types of cells and tissues secreting liquids with different ecological func-
tions, not always related to plant reproduction and dispersal. Analogies
between nectary and other secreting tissues are more evident when the secre-
tion is liquid and “exported” outside the organ. In certain flowers elaiophores
may produce a reward rich in lipids (Vogel, 1988), and osmophores produce
a fragrance attractive to animals (Effmert et al., 2005). Vegetative parts,
mainly leaves, of plants living in wet environments may have hydathodes,
structures that passively secrete water and excess mineral ions from xylem
vessels by a process known as guttation (Feild et al., 2005). According to
Feild et al. (2005), this process may be defensive in that it prevents flooding
of the mesophyll. Carnivorous plants have modified leaves covered with
various glands which function in attracting and digesting the prey (Joel,
1986). Pitcher plants (
Sarracenia
and
Nepenthes
)
have large extrafloral nec-
taries above the pitcher (Dress et al., 1997; Owen & Lennon, 1999). All
these types of secretory cells have been considered and analysed from an
anatomical point of view (reviewed by Fahn, 2000). The structural similarity
between nectaries, hydathodes, and elaiophores has often been noted
(Schmid, 1988; Vogel, 1997).
Sugary secretions are also produced by fungi and insects.
Fungal infection
of the ovary of
Secale cereale
(Poaceae) attacked by
Claviceps purpurea
(Ascomycetes) elicits production of a sugary fluid that the parasite exploits to
disperse its conidia (Alexopoulos et al., 1996). The comparison with nectaries
is valid from an ecological point of view, because the pathogenically induced
