Merithallus

In fact, in the same way that we see in the monocotyledonous embryo, which has taken all its normal expansion, a radicular mamilla which constitutes its descending system, and a cauliculus, a cotyledon, and its support, which form together its ascending system, in the same manner also we see in the more advanced plant the root which represents the radicle, that is to say, the descending system, and the merithallus with the leaf and its petiole, which represent the cauliculus, the cotyledon, together with its support, that is to say, the ascending system.

[…]but when we examine the increase of this branch in part, we see, with Cassini, that each merithallus, or internode, grows principally by its lower part[…]

Prof. F. Delpino regards the leaf, not as an independent organ, but as a section of a merithallus, the free projecting portion of a cone of growth, the remaining elements of which are closely united with one another to form the axial shoot.