Orbis caeruleus

The Fibonacci is a large fern-like plant that is related to a Terran fiddlehead and has the same fiddle neck appearance when the leaves are young. The common name of the plant is based on the resemblance of the coiled juvenile leaf to the famous Fibonacci spiral, which is based on the ancient number sequence in which each number is the sum of the two preceding numbers. Botanists on Pandora have recognized this mathematical pattern in the Fibonacci and many other botanical specimens. (Read more)
Like the fiddlehead, the Fibonacci leaves contain algae, although in this case they are blue-green in color and very useful to the plant. These symbiotic algae are able to perform photosynthesis to make their own food. They also capture nitrogen from the air and fix it into nitrate, which is usable by the plants. This allows the Fibonacci to grow in poor soils with little nutrition. The Na’vi collect the Fibonacci to extract the blue pigment for use in paints.

The toxin in the algae is used by the Na'vi to create a poison that can be used for the tips of arrows or spears.