adventitious

Concise Oxford English Dictionary © 2008 Oxford University Press:
adventitious /ˌadv(ə)nˈtɪʃəs/

▶adjective

• 1 happening according to chance.

• 2 coming from outside; not native.

• 3 Biology formed accidentally or in an unusual position.

• 4 Botany (of a root) growing directly from the stem or other upper part of a plant.

– derivatives
adventitiously adverb.

– origin C17: from L. adventicius ‘coming to us from abroad’ + -ous (see also -itious2).