PHYSICAL FACTOR RESULTS 97 
exclusion of single curves. Combinations are made simply by tracing the 
curves to be compared upon the same sheet, it being understood that dis-. 
similar curves, e. g., level and station, can not be combined. Colored inks 
_are an absolute necessity in combining; the primary principle underlying 
their use is that curves that approach closely or cross each other must be 
traced in inks that contrast sharply. As elsewhere stated, it has been made 
the invariable rule to use the same color for the same level or point. This 
applies especially to temperature, but holds also for humidity, light, wind, 
and water-content, so that the color always indicates the level. For the 
same reason, it is applied to a combination of point curves for one station, 
though it is inapplicable to a series of point curves when these lie in the 
same level. Light readings above 6 feet and water-content readings below 
15 inches necessitate the use of additional colors. 
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 correspond- 
ing 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 com- 
binations 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 in- 
teresting 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 instruc- 
tive 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 
