Ants on the Plants

Saw these large ants tending to these brown patches on these broad leaves. OK...does that sentence give any useful information? No...except to a biologist I'm sure it very clearly points out I know nothing about plants or ants. I had an Uncle Milton ant farm when I was a kid, but I never saw anything like this. Can anyone shed some light on this one?

There are several good ant identification sites on the web. A few examples are Ant Web, Bug Guide (useful for more than just ants), and these photos from the MS State site. I learned a little about ants. The most important thing I learned is there are a heck of a lot of ant species out there! From the lack of ocelli, the 12 segmented antenna, the smooth arched back, and the single segment pointed me to some type of Camponotus -- a carpenter ant. However, from descriptions it doesn't make sense that I'd see them tending to stuff on the top of broad plant leaves. And what are they working on? They were continuously touching one and then the next with their antenna. I'm hoping some myrmecologist will leave a comment.