=160. Coefficient of transpiration.= At present it does not seem feasible to express the transpiration of a plant in the form of a definite coefficient, but it is probable that the application of exact methods to each part of the problem will finally bring about this result. Meanwhile the following formula is suggested as a step toward this goal: _t_ = _g_(_u/l_)_LHT_, in which _t_, the transpiration relation of a plant, is expressed by the number of grams of water lost per hour, on a day of sunshine, by one square decimeter of leaf, considered with reference to the stomata of the two surfaces, and the amount of the controlling physical factors, light, humidity, and temperature, at the time of determination. For _Helianthus annuus_, this formula would appear as follows: _t_ = 2(²⁰⁰⁄₂₅₀) : 1 : 50 : 75°. To avoid the large figures arising from the extent of surface considered, the number of stomata per square decimeter is divided by 10,000. This amounts to the number per square millimeter, and time may consequently be saved by using this figure directly. While this formula obviously leaves much to be desired, it has the great advantage of making it possible to compare ecads of one species, or species of the same habitat or of different habitats, upon an exact basis of factor, function, and structure. _ADAPTATION_ =161. Modifications due to water stimuli.= In adaptation, the great desideratum is to connect each modification quantitatively with the corresponding adjustment. This is even more difficult than to ascertain the quantitative relation between stimulus and functional response, a task still beset with serious obstacles. At the present time, little more can be done than to indicate the relation of marked adaptations of organs and tissues to the direct factors operating upon them, and to attempt to point out among the functions possibly concerned the one which seems to be the most probable connection between the probable stimulus and the structure under investigation. In the pages that follow, no more than this is attempted. The general changes of organs and tissues produced by water are first discussed, and after this is given a summary of the structural features of the plant types based upon water-content.