Biology Reference
In-Depth Information
systematics. In some plant families the nectaries differ greatly in position,
morphology, and nectar composition, while others are relatively homogene-
ous. Species of either small or large families may resemble each other in
nectary organization but others, such as Ranunculaceae, show wide variation.
Nectary structure and nectar production (Chapters 3 and 4)
Nectary structure may vary with nectary position in the flower. Though nec-
tary histological components have long been recognized, ultrastructural
studies of secretory cells are revealing new details of organelles during nec-
tary development and secretion. Independently from the anatomical
organization, the extent of nectary parenchyma determines the quantity of
nectar produced and hence the type of pollinator. The anatomical diversity of
nectaries may be matched by a similar diversity in the mechanisms of nectar
secretion and presentation. The concept of secondary nectar presentation,
namely when nectar is not exposed close to the nectary but elsewhere in the
flower, as in spurs, was recently developed.
Nectar components may be produced and elaborated in different parts of
the nectary tissue. It is generally assumed that nectar carbohydrates are de-
rived from phloem sap, but photosynthesis in the nectary parenchyma may
be an important supplementary source of carbohydrates. The storage of
starch in non-photosynthesizing nectaries is an advantage when rapid pro-
duction of nectar is required. We discuss the dynamics of nectar production,
including reabsorption of unconsumed nectar. The interaction between the
dynamics of nectar production by the plants and nectar feeders defines the
nectar standing crop. Animal-plant interactions also affect the site and man-
ner of nectar presentation. Variability in nectar quantity and quality is
apparent at many different spatial and temporal levels and is strongly af-
fected by environmental parameters.
Nectar chemistry and molecular biology (Chapters 5 and 6)
The chemical complexity of nectar has been apparent since the prolific work
of Herbert and Irene Baker, but analytical methods have naturally improved
and a new look at nectar chemistry is appropriate. Nectar sugar composition
has been extensively studied, particularly the relative proportions of sucrose,
glucose and fructose, and the data have pointed to convergence in nectar
characteristics of unrelated plant species and pollinator type (Baker & Baker,
1983); however, the fact that there are phylogenetic constraints on the adap-
tation of nectar to pollinators has become more apparent in recent years.
Nectar is also much more than a dilute sugar solution, and there is renewed
