34 



PLANT PHYSIOLOGY AND ECOLOGY 



Oak-hickory forest. 

 April 20, 1901. Clear. Southeast wind. 



46. Kinds of curves. The most graphic way of bringing out 

 the factor differences between habitats or stations is by means 

 of curves. The factors that lend themselves most readily to this 

 method are the variable ones, water content, humidity, light, 

 temperature, and wind. Curves representing these are spoken 

 of respectively as water-content curves, humidity curves, etc. 

 With regard to the time and position of the readings upon which 

 they are based, they are divided into level, station, and point 

 curves. A level curve is one based upon readings made at the 

 same level through a series of stations, e.g., the level curve of 

 surface temperature. The station curve shows the variation of a 

 factor through the different points at which readings are made in 

 a single station. The point curve has for its basis the hourly or 

 daily variation of a factor at a single point in a station, such as the 

 variation in humidity during the day at the surface of a barren 

 ridge. All of these may be simple curves when based upon a single 

 reading through a level, station or day. or mean curves when they 

 are based upon the average of a number of such readings. 



47. Combinations of curves. Curves are often combined in 

 order to permit of a ready comparison between them. Com- 

 bination is brought about by tracing upon the same sheet the 

 curves to be compared. Dissimilar curves, e.g., level and sta- 

 tion, can not be combined. Colored inks are an absolute necessity 

 in making combinations. The principle underlying their use 

 is that curves which approach closely or cross each other must 

 be traced in inks that contrast sharply. It is important to use 

 the same color invariably for the same level or point. The variety 



