Root Nodules


L. to R. - Casuarina glauca nodule from Tunisia (about 15 cm diameter; Hafedh Nasr)

A. incana subsp. rugosa nodule from Connecticut (about 1.5 cm diameter; D. R. Benson)

Casuarina nodules (about 1 cm diameter; R. H. Berg)

Morella pensylvanica nodule (about 5 cm diameter; D. Benson)

Nodule Appearance

Actinorhizal nodules appear in a variety of forms in the field. They all resemble clusters of lateral roots that increase in size by repeated branching of the nodule lobe tips, leading to the final structures shown above. Some nodules have negatively geotropic nodule roots growing from the tips of nodule lobes, such as in the M. pensylvanica shown above, whereas others may be supplied with lenticels on the surface of the nodule, presumably to enhance gas exchange.

Nodules have a range of colors from light yellow to orange, to various hues of brown and even red. In size, they range from less than a millimeter for very young nodules up to more than ten centimeters for older nodules that have developed in loose sandy soil. The larger nodules (more than 1 cm) generally only have active nitrogen-fixing tissue on the tip of young nodule lobes. The older portions of the nodule in the interior of the cluster is frequently quite senescent and inactive. Nodules are perennial and can continue to develop from season to season over several years.

The position of the nodules on roots varies with the plant and environment in which the plant lives. For example, alder trees growing near water tend to bear nodules quite near the surface such that merely brushing away the leaf litter beneath an alder plant reveals dozens of nodules. By contrast, some of the rosaceous and rhamnaceous species that dwell in dry rocky soil or chaparral tend to have nodules that are quite deep and may be difficult to find in rocky soil.

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