Nitrogen is a limiting resource for plants. It is essential for life functions and yet they do not produce it on their own. Instead, plants need to get it from their environment. They cannot uptake gaseous nitrogen, which is a shame because it makes up 78.09% of our atmosphere. As such, some plants have developed very interesting ways of obtaining nitrogen from their environment. Some, like the legumes, produce special nodules on their roots, which house bacteria that fix atmospheric nitrogen. Other plants utilize certain species of mycorrhizal fungi. One family of plants, however, has evolved a symbiotic relationship that is unlike any other in the angiosperm world.
Meet the Gunneras. This genus has a family all to itself - Gunneraceae. They can be found in many tropical regions from South America to Africa and New Zealand. Some species of Gunnera are small while others, like Gunnera manicata, have leaves that can be upwards of 6 feet in diameter. Their leaves are well armed with spikes and spines. All in all they are rather prehistoric looking. The real interesting thing about the Gunneras though, is in the symbiotic relationship they have formed with cyanobacteria in the genus Nostoc.
Gunnera produce specialized glands that house these cyanobacteria. The glands are filled with a special mucilage that not only attracts the cyanobacteria, but also stimulates it to grow. Once inside the glands, the cyanobacteria begins to grow into the plant, eventually fusing with the Gunnera cells. From there the cyanobacteria earn their keep by producing copious amounts of usable nitrogen and in return, the Gunnera supplies carbohydrates. This relationship is amazing and quite complex. It also offers researchers an insight into how such symbiotic relationships evolve.