Combinations may be made of the curves of a single factor for purposes of comparison, or they may consist of curves of different factors in order to aid in interpreting or indicating their relation to each other. Curves of the same factor may be combined to form various series. The level series consists of all the level curves for the stations under observation, e. g., the six levels for temperature, three levels for wind, etc. Similarly, the station series is a combination of all the station curves, and a corresponding arrangement may be made for point curves with reference either to station or to level. An extremely valuable combination of curves is that of the holard and chresard for a series of stations. The most important combinations of the curves of different factors are naturally those based upon factors intimately related to each other or to the plant. The grouping of water-content and humidity curves is of great value, especially when the transpiration curve is added. Light and temperature curves make an interesting combination, while a humidity, temperature, and wind series is of much aid in tracing the connection between these factors. Finally, it is altogether feasible to arrange the curves of water-content, humidity, light, temperature, and wind upon the same sheet in such fashion as to give a graphic representation of the whole physical nature of a single habitat or a series. In all combinations of curves representing different factors, it must be borne in mind that the position of a curve does not represent a definite value with reference to the others, since some are based upon per cents, others upon degrees, etc. The comparison must be based upon the character of the curves, but even then it is an important aid. An instructive grouping has been employed where series of readings on the same day, or on two successive days in forest and in prairie have yielded the usual level series of curves. The series for the two habitats are arranged on the same page, one at the right and the other at the left, and permit direct comparison of corresponding level or factor curves, both with respect to position and character.