In the present paper the morphology and development of the leaves of the Gramineae are considered. Bugnon's criticism (13) is discussed, and it is concluded that, as in the case of the leaves of other monocotyledonous families, the grass leaf is best interpreted as a phyllode consisting of a sheathing leaf base and a petiolar limb. It is shown that the leaf of the Gramineae cannot be treated as an isolated type, but that there are striking parallelisms between the leaf of this family and of other monocotyledons; this parallelism comes out not only in the mature form, but also in the fact that invagination plays a conspicuous part in the ontogeny of the limb. A study of the prophyll confirms the theory of Bugnon and others, that this organ is a single leaf, rather than a pair of foliar members in a state of congenital fusion. Bugnon's recent criticisms of the views on the morphology of grass seedlings put forward in 1915 by Miss Sargant and the writer are considered. He treats the scutellum and coleoptile ...