surfaces which each leaf displays before it unfolds to the light, but which becomes more marked when it has unfolded.[47] From the ordinary cases let us now pass to the exceptional cases. We will look first at those in which the two faces of the leaves differ but little, or not at all--their circumstances being similar or equal. Leaves that grow in approximately-upright attitudes, and attitudes which do not maintain the relative positions of the two surfaces with constancy, may be expected to display an unusual likeness between the two surfaces; and among them we see it. The Grasses may be named as a group exemplifying this relation; and if, instead of comparing them as a group with other groups, we compare those dwarf kinds of them which spread out their leaves horizontally, with the large aspiring kinds, such as _Arundo_, we trace a like antithesis: in the one the contrast of upper and under is very obvious, while in the other it is scarcely to be detected. Leaves of various other Monocotyledons that grow in a similar way, similarly show us a near approach to uniformity of the two surfaces; as instance the genus _Clivia_ and the thinner-leaved kinds of _Yucca_. Where the contrast of upper and under is greatly diminished by the assumption of a rounded or cylindrical form, instead of a flattened form, the same thing happens. The genus _Kleinia_ furnishes illustrations. It may be remarked, too, that even within the limits of this genus there are instructive variations; for while in _Kleinia ficoides_ the leaves, shaped like pea-pods, are broadest in a vertical direction, and have their lateral surfaces alike in conditions and structure, in other species the leaves, broader horizontally than vertically, exhibit unlikeness between the upper and under sides. Equally to the point is the evidence furnished by vertically-growing leaves that are cylindrical, as those of _Sanseviera cylindrica_, or as those of the Rush-tribe: the similarly-placed surface has all around a similar character. Of kindred meaning, and still more conclusive, are the cases in which the under side of the leaf, being more exposed to light than the upper side, usurps the character and function of the upper side. If a common Flag be pulled to pieces, it will be seen that what answers to the face in other leaves, forms merely the inside of the sheath including the younger leaves, and is obliterated higher up. The two surfaces of the blade answer to the two under halves of a leaf that has been, as it were, folded together lengthways, with the two halves of its upper surface in contact. And here, in default of an upper surface, the under surface acquires its character and discharges its function. A like substitution occurs in _Aristea corymbosa_; and there are some of the Orchids, as _Lockhartia_, which display it in a very obvious way.