I walked out into my front yard the other day and took a close look at the branches of one of our Japanese Cherries (some kind of Prunus serrulata), which has decided to grow out over our walkway (speaking of "prunus"...). As I only ever studied one species of cherry (serotina) back at school, I took a closer look at the leaves and noticed these little green bumps at the base of the leaf:
As I craned in to look closer, a large ant walked up to it and plucked off a big drop of liquid that had collected in that little brown pore in the center. I realized that these be must nectar producing structures that the plant was using to attract insects. Usually, plants will have nectar glands ("nectaries") in their flowers to attract pollinators, but often plants will use nectar to recruit contractors that perform other kinds of services. Classic examples of this are the several species of acacia tree which have evolved to serve as homes for a particular species of ant that will attack anything that comes in contact with the plant. In return for this defense, the acacia provides the ants with hollow thorns to live in, proteinaceous buds, and nectar.
Being quite familiar with the attentiveness with which acacia ants defend their home-trees, I quickly decided to test this by grabbing the plant and trying to attract the ant's attention. No response. It made sense, since I didn't see a colony of ants living in close association with this tree; this ant was on its own, as were the solitary bees and wasps taking advantage of these nectar-wells. A little internet research revealed that the timing of these nectaries is apparently tied to the emergence of caterpillars, and the attention that they attract from other insects increases the chances that the Cherry's pests will also be taken as prey.
This is an interesting situation, because unless the Cherry brought all its insect bouncers with it from Japan, this non-native ornamental tree is associating with all kinds of native insects. Apparently it still works to attract defenders, but how effective are these defenders? Does the plant need them in this setting? From what are the insects' attentions being diverted? This small situation brings to mind a whole suite of issues that come into play when looking at the ecological effects of species that have taken over in places where they didn't evolve.
For more on this, see BugBlog's post on the subject: http://abugblog.blogspot.com/2011/06/nectar-from-cherry-leaves.html. To see the photos in full, see here: http://www.morrisonmast.com/p986799541