HERE. Today, I trotted outside for a brief 15-20 minute photographic interlude, and was rewarded with something rather cool.
Ohio Prairie Nursery, is this little gem. It's Partridge Pea, Chamaecrista fasciculata, a showy pinnate-leaved beauty. It is easy to grow, quick to arise, and pleasing to the eye. Insects also find it pleasing to the palette, as we shall see.
We are not here, in this photo, to look at this ant. We'll look at ants in a bit. I draw your attention to the tiny cuplike appendage on the leaf's petiole, upper lefthand corner of the photo. Note the glistening reddish syrup within. The petioles (leaf stalks) of all the leaves are similarly adorned with these cups, which are known as extrafloral nectaries. A great many flowering plants are laden with intrafloral nectaries - sweet nectar rewards hidden within the flowers. They are there to entice pollinators into the bloom, where they will be dusted with pollen and thus complete the plant's pollination process.
Extrafloral nectaries are far scarcer, and arguably far more interesting.
The impact of this ant army? An incredibly effective deterrent to any wannabe defoliators such as caterpillars or other insects that would damage the plant. If the ants detect a threat to their spoils, they will launch a brutal attack and drive off or kill the interloper. This behavior, of course, greatly benefits the plant and is an excellent example of a mutualistic relationship: both organisms in the partnership benefit.
Plants protect themselves in numerous ways: production of various compounds that ward off herbivores; thorns; development of difficult to digest fibrous products such as lignin, etc. But few protective strategies top the complex active defense system brought on by development of extrafloral nectaries. This defense system isn't particularly common; for instance, of the 330 or so species of Chamaecrista worldwide, only about 26 have evolved extrafloral nectaries.
Partridge Pea is in full bloom now. If you're around some of these plants, have a look to see if ants are working the extrafloral nectaries.