It is difficult to tell whether a plant with large leaves exposes more surface than one with small leaves. Structural modifications tending to reduce resistance within the leaf to the flow of water may be as important for xerophytes as features leading to a reduction of transpiration. In the case of dissected leaves with narrow segments and of pinoid, ericoid, and cupressoid leaves, no part of the mesophyll is morq than a short distance from the channels of supply. Storage of water in leaves is of secondary importance in many cases. The extent of the internal surface of the leaf is of considerable importance, since the greater part of the resistance to diffusion is located there.