The modifications of the stomata with respect to transpiration are numerous, yet all may be classed with reference to changes of number or level. With the exception of aquatic and some shade plants, the number of stomata is normally greater on the less exposed, i. e., lower surface. The number on both surfaces decreases regularly as the danger of excessive water loss increases, but the decrease is usually more rapid on the upper surface, which finally loses its stomata entirely. It has been shown by many observers that species growing in dry places have fewer stomata to the same area than do those found in moist habitats. This result has been verified experimentally by the writer in the case of _Ranunculus sceleratus_, in which, however, the upper surface possesses the larger number of stomata. Plants of this species, which normally grow on wet banks, were grown in water so that the leaves floated, and in soils containing approximately 10, 15, 30, and 40 per cent of water. The averages for the respective forms were: upper 20, lower 0; upper 18, lower 10; upper 18, lower 11; upper 11, lower 8; upper 10, lower 6. Reduction of number is effective, however, only under moderate conditions of dryness. As the latter becomes intense, the guard cells are sunken below the epidermis, either singly or in groups. In both cases, the protection is the same, the guard cells and the opening between them being withdrawn from the intense insolation and the dry air. The sun rays penetrate the chimney-shaped chambers of sunken stomata only for a few minutes each day, and they are practically excluded from the stomatal hollows which are filled with hairs. The influence of dry winds is very greatly diminished, as is also true, though to a less degree, for leaves in which the stomata are arranged in furrows. Sunken stomata often have valve-like projections of cuticle which reduce the opening also. Finally, in a few plants, water loss in times of drouth is almost completely prevented by closing the opening with a wax excretion. 5. _Modifications in the chlorenchym._ A decrease in the size and number of the air passages in the leaf renders the movement of water-laden air to the stomata more difficult, and effects a corresponding decrease in transpiration. The increase of palisade tissue, though primarily dependent upon light, reduces the air-spaces, and consequently the amount of water lost. The development of sclereids below the epidermis likewise hinders the escape of water. Finally the character of the cell sap often plays an important part, since cells with high salt-content or those containing mucilaginous substances give up their water with reluctance.