Mutualism in Bluebonnets

I found some bluebonnets growing on campus last week and decided to plant one in a container in the greenhouse. As I was repotting it, I noticed that this particular specimen had a few nodules in its root system. 

Note the three nodules indicated by the arrows

These nodules contain Rhizobium bacteria, which can "fix" nitrogen from the gaseous form in the atmosphere to the mineral form that is usable by plants. This is a prime example of mutualism in nature - the plants gain access to the nitrogen from the bacteria, and the bacteria have a safe place to live in the bluebonnet's root system. Of course, bluebonnets don't always form nodules; sometimes, gardeners have to introduce the bacteria via inoculants.