adventitious

adjective

(L. adventicius: fortuitous, coming from outside, unexpectedly) Plant structures or tissue occurring sporadically in an abnormal position, i.e. in a position other than the usual location, i.e. coming from another source, i.e. not inherent or innate, e.g. roots arising from the shoot system, from stems or leaves, buds arising elsewhere than in axils of leaves, rhizoids arising from leaf or costal cells.