2. _The floating type._ With respect to form and the structure of the upper part of the leaf, floating leaves are essentially similar to those of amphibious plants. They are usually lacquered or coated with wax to prevent the stoppage of the stomata by water. Stomata, except as vestiges, are found only on the upper surface, and the palisade tissue is much less developed than the sponge, which is uniformly characterized by large air-chambers. The stems are elongated, the aerating system is enormously developed, and the supportive tissues are reduced. In the _Lemnaceae_, the leaf and the stem are represented by a mere frond or thallus, and the roots are in the process of disappearance, e. g., _Spirodela_ has several, _Lemna_ one, and _Wolffia_ none. 3. _The submerged type._ Both stem and root have been greatly reduced in submerged plants, owing to the generalization of absorption and the density of the water. The leaves are greatly reduced in size and thickness, chiefly, it would seem, for the purpose of insuring readier aeration and great illumination. The leaf may be ribbon-like, linear, cylindrical, or finely dissected. Stomata are sometimes present, but they are functionless and vestigial. A distinction into palisade and sponge tissues, when present, must also be regarded as a vestige; the chlorenchym is essentially that of a shade leaf. The air-chambers are much reduced, and sometimes lacking; they function doubtless as reservoirs for air obtained from the water. PHOTOHARMOSE _ADJUSTMENT_