Plants may contain nitrifying bacteria in their tissues from which they get usable nitrogenous chemicals or absorb the chemicals discharged into the soil from other decomposing dead plants. Acacias usually obtain usable nitrogen in both ways. Root nodules, an anatomical modification of the plant root, caused by these specific bacteria, are the result of a symbiotic association between the plant and bacteria. The bacteria take in atmospheric nitrogen and by use of an enzyme make it available to the host plant. The host makes sugars available to the bacteria in return.
Sugars are high energy carbon compounds used in cell growth and function.
The bacteria attach to the hair on the concave side. Growth is arrested at the root tip and transferred to the attachment point. This grows to the base of the hair and becomes meristematic, ie. causes a zone of frequent cell division in an orderly manner but not so well defined as in a growing bud. The root nodules on the roots of an Acacia can often be seen by removing a small plant from its container. The dividing cells grow in all directions and form a spherical nodule in which a vascular system develops.