proteoid root A root (PO:0009005) that forms one or more clusters of closely spaced lateral roots (PO:0020121) of limited growth along its length. plant_anatomy root cluster (related) austinmeier PO:0000067 The terms proteoid root, cluster root, and root cluster have all been used in the literature to describe either the proteoid root axis or clusters of rootlets, and caution should be taken when comparing studies because the definitions of these terms vary (Watts and Evans, 1999; Plant Physiology). cluster root (related) PO_GIT:588 2015-08-11T15:05:12Z root