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.