Biology Reference
In-Depth Information
Nectaries are specialized tissues that secrete a sugary solution involved in
interactions with animals. The term does not indicate a uniform or well-
defined anatomical structure, however (Fahn, 1979; Pacini et al., 2003).
There are various types of nectary, situated anywhere in the flower and in
widely different parts of plants, with different origins and types of organiza-
tion. The diversity of nectaries is evident in Vogel's exhaustive description
of the types and structures of nectaries in many angiosperm families (Vogel,
1997, 1998a,b,c). In general terms, nectaries consist of three components
(Fahn, 2000; Pacini et al., 2003):
•
An epidermis, with or without stomata and trichomes, where nectar is
released to the exterior.
•
Specialized parenchyma that produces or stores nectar solutes.
•
The vascular bundle that conveys water and nutrients to the parenchyma.
The conventional view that nectar originates from phloem sap but may be
modified by the nectary parenchyma is perhaps oversimplified. The sugar
component of nectar is derived from photosynthesis by the nectary itself, or
probably more commonly by photosynthesis in other parts of the plant, with
or without starch as an intermediary storage product. At least some extra-
floral nectaries, which secrete for prolonged periods, photosynthesize. The
breakdown of stored starch makes high rates of nectar production possible, at
any time of the day (Pacini et al., 2003).
2
SECRETIONS ANALOGOUS TO NECTAR
The floral secretions mentioned by Endress (1994a) as potential early re-
wards to pollinating insects were pollination drops and stigmatic secretions.
The exposed ovules of gymnosperms secrete a sugary fluid at the micropylar
end, and this so-called pollination drop acts as a nectar reward for insects in
Gnetum
,
Ephedra,
and
Welwitschia
(Owens et al., 1998; Gelbart & von
Aderkas, 2002). Strong evolutionary, cytological, and chemical similarities
exist between the pollination drop and nectar. Both are liquids containing
carbohydrates and proteins. The function of the pollination drop is to rehy-
drate pollen and to serve as germination medium, and rehydration of pollen
grains is only possible if the sugar concentration of the drop is relatively
low: more concentrated solutions would tend to dehydrate it. Retraction of
the pollination drop draws pollen into the ovule, whereas nectar is consumed
by animals visiting the flower and may be reabsorbed if not collected by
visitors. Pollination drops always retract to draw pollen into the micropyle,
whereas reabsorption of unconsumed nectar only occurs in some species,
