Pretty petals in various colors and arrangements are a major part of how flowering plants attract animals as pollinators.
Another part of the attraction game in most flowers is the offering of a food reward, usually nectar, although some flowers (such as Calypso orchids) just look like they would have nectar and fool naïve insects into making pollinating visits.
Structure of the flower often determines what animals can obtain the nectar. Simple, saucer-shaped flowers (e.g., salmonberry, or a buttercup) are open to any hungry visitor, although some may be better pollinators than others. Some flowers are more specialized and complex. For example, columbines keep the nectar reward in long spurs at the “back” of the flower, away from the entrance. Only hummingbirds or long-tongued bees can reach that nectar from the entrance, and in so doing, they contact the sexual parts of the flower and achieve pollination. (Of course, some animals have figured out how to rob the flower of its nectar, without pollinating; short-tongue bees bite the end of the nectar spur and steal the nectar).
Nectar is not just sugar water, although that’s what most of us put in our hummingbird feeders. Nectar composition is highly variable, depending not only on species of flower, but also age of the flower and growing conditions of the plant, the availability of water, humidity, microbial contamination and probably other factors too. Sugar concentration varies, and so do the proportions of different sugars. The principal sugars are sucrose (composed of one glucose molecule and one fructose) and its two component sugars, the proportions varying with how much of the sucrose has been hydrolyzed. Other sugars can be present in trace amounts. Flowers that are pollinated by different animals (bees, birds, bats, butterflies, etc.) commonly produce sugars that differ in concentration and composition. Some South African flowers produce a totally different sugar (xylose), which is avoided by insect and bird visitors but utilized by rodents that are the pollinators of those flowers!
Many nectars contain a variety of amino acids; within a species, the concentration of amino acids usually varies more than the composition. Some amino acids are needed for protein-building; some have other functions, such as defense against microbes. Amino acids tend to be especially rich in butterfly-pollinated flowers, so researchers wondered if butterflies preferred nectars with amino acids over those that had none; in some cases, they do. Furthermore, some female nectar-feeding butterflies that sipped nectar with amino acids had higher egg production than females feeding on plain sugar-water, but this was not true for all species examined. In one case, nectar amino acids also increased male reproductive success: the offspring were larger, apparently because of material given by the male to his mate during the mating process.
Other insects often prefer nectar with amino acids too: mosquitoes, for example. Female mosquitoes of a certain species lived longer when their nectar diet contained amino acids as well as sugar; but no such effect was seen in males of those species. At least in some circumstances honeybees prefer nectar with amino acids. Some flies favor nectar with certain amino acids, which tend to occur in high concentrations in fly-pollinated flowers.
A few kinds of floral nectar contain lipids, fatty acids that can be used as fuel by a visiting animal. Many contain vitamins. Some nectars even contain alkaloids and other distasteful substances that may deter some potential flower visitors.
Not all nectar is found in flowers, however; in many kinds of plants, there are extrafloral nectaries (meaning “outside the flower”), which appear on leaves or stems. The nectar in these nectaries is generally quite different from that in flowers. Amino acids are present, but they are usually the non-protein-building kinds.
Several studies have shown that ants often (but not always) prefer extrafloral nectar that contains amino acids over nectar that lacks them and are attracted to these nectaries. Sometimes, parasitic wasps and predatory flies are, too. Many studies have shown that the ants (and wasps and flies) prey on insects that feed on leaves or stems and thus typically reduce the amount of damage that the herbivores do. It’s a mutualistic defense system: the plant pays the ants to defend it, and the ants get food.
Several local species (including species of Populus, Viburnum, Rosa, and Impatiens) may have extrafloral nectaries, but not all individuals may exhibit them; in some species, the trait is heritable but not all genetic families have the trait. Furthermore, in some cases the extrafloral nectaries are only produced after some herbivore damage has occurred. In cottonwoods and their relatives (aspens), expression of these nectaries is reportedly both heritable and inducible. Elderberry shrubs have them too; they are the little stalks near the axils of the fresh leaves in the accompanying photo; nectar is borne at the top of the stalks. However, so far, we have no observations of insect visitors to these nectaries.
The most-studied extrafloral nectaries in non-tropical plants may be those found on bracken fern, a cosmopolitan species that grows in Southeast. (Ferns do not have flowers, so the nectaries are necessarily extrafloral, but the term is used for convenience of comparison with the truly extrafloral nectaries of flowering plants.) The nectaries on bracken are produced on very young, developing fronds, before the leaves expand, in late spring and early summer. They are dark blobs at the junctions of the major pinnae or leaflets, and small ones may occur along the midribs of each pinna. A study in Britain showed that the size of bracken nectaries varied with habitat: they were larger in open habitats than in woodland. The numerous studies of bracken nectaries have produced highly variable results: some showed that ants defend the young fern particularly against sucking insects, or only insect herbivores in the act of egg-laying, or just the eggs of the herbivores, or only leaf-chewing herbivores, while others showed no effect of the ants attracted to the nectaries. Presumably, habitat effects on nectary size are likely to be reflected in ant activity, and furthermore, the deterrent effect of ants depends on what species of ants come to the nectaries and the density of the ants. It also matters just what herbivores are involved: some insect herbivores are very well defended against ant attack and are impervious to their assaults. A lesson in ecological complexity!
I have the untested impression that extrafloral nectaries on our local plants are not very well developed. For instance, on cottonwoods, although I find yellow, sticky exudates, probably a resin that might deter leaf-eating insects, on young stems near where new leaves attach, I do not see nectar-secreting glands in the typical locations near the junction of leaf blades with petioles. Here in Juneau, we’ve seen no visitors to elderberry extrafloral nectaries, and there are hardly any ants in the places where we’ve seen other species that may have extrafloral nectaries. Perhaps this plant/animal mutualism does not work well here?