Cells and other organisms
Mycorrhizal symbiosis
Trees, as we well know, interact with all kinds of creatures, from squirrels munching on acorns to birds perching on branches. But they also interact at scales invisible to the naked eye. A tree attacked by a parasitic fungus defends itself by strengthening the cells around the infection. It fills them with lignin, the molecule that gives trees their rigidity. The fungus will have to get past these tougher cells blocking it before it can penetrate farther into the tree. And then there are other trees that deform their root cells when their roots are colonized by nitrogen-fixing bacteria. As the cells grow, they end up absorbing these helpful bacteria.
Friendly fungi
They may be out of sight and hard to reach, but roots know how to work together. Almost all trees benefit from one or more kinds of symbiotic relationships with fungi called mycorrhizae, which help make their root systems much more efficient. Sometimes the tree will even allow its partner to colonize its root cells. This arrangement is called endomycorrhizal symbiosis (from Greek, endo meaning inside).
