Rachis

Rachis, now sometimes spelt, more strictly in the Greek form, rhachis, is an axis of any kind; but is used most generally for the prolongation of the leaf-stalk or petiole in a compound leaf beyond the lowest leaflets, thus corresponding to the midrib of a simple leaf. A simply pinnate (q.v.) leaf will have but one rachis; but in a bi-pinnate leaf there are secondary rachides as well as the primary one; and there may similarly be tertiary rachides.