Delphinidin

Although almost 80% of our atmosphere consists of nitrogen gas, plants cannot convert the nitrogen gas to usable forms. A few species of bacteria, however, produce enzymes with which they can convert nitrogen into nitrates and other nitrogenous substances readily absorbed by roots. Members of the Legume Family (Fabaceae), which includes peas, beans, alfalfa, and a few other plants such as alders, form associations with certain soil bacteria that result in the production of numerous small swellings called root nodules that are clearly visible when such plants are uprooted.
The nodules contain large numbers of nitrogen-fixing bacteria. A substance exuded into the soil by plant roots stimulates Rhizobium bacteria, which, in turn, respond with another substance that prompts root hairs to bend sharply. A bacterium may attach to the concave side of a bend and then invade the cell with a tubular infection thread that does not actually break the host cell wall and plasma membrane. The infection thread grows through to the cortex, which is stimulated to produce new cells that become a part of the root nodule; here the bacteria multiply and engage in nitrogen conversion.

Root nodules should not be confused with root knots, which are also swellings that may be seen in the roots of tomatoes and many other plants. Root knots develop in response to the invasion of tissue by small, parasitic roundworms (nematodes). Unlike bacterial nodules, root knots are not beneficial, and the activities of the parasites within them can eventually lead to the premature death of the plant.