36 PLANT PHYSIOLOGY AND ECOLOGY 



traced in ink. Each curve sheet is properly hibeled, and 

 such explanatory notes as are desirable are written upon the 

 back. 



49. Intervals for the different factors. In practice, the in- 

 tervals and ranges of the curves of the other factors have been 

 arbitrarily fixed, as in the case of temperature. For water-con- 

 tent curves each square represents a value of 2%, the smaller 

 squares being 0.4%, and the range 2-48%. The unit value for 

 humidity curves is taken as 5 percent, thus giving room on the 

 sheet for the entire range from 1-100 percent. When a hand 

 anemometer is used, curves of wind velocity are based upon the 

 number of feet per minute. One hundred feet is taken as the 

 unit value, and the range is from 0-2200 feet. The unit value 

 for the curve of light intensity is .05.- Each small square is .01, 

 which permits a range of .01 to 1. on a sheet. Consequently, 

 in plotting the curve of a series of habitats with a range in in- 

 tensity greater than this, it is necessary to paste two sheets to- 

 gether end to end. This is the usual device when the range of 

 curves is too great, except when the excess is slight. In this 

 case, the curve is left open at the top, and the maximum value 

 is indicated at one side. All curves in combination are labeled 

 at the beginning or left to indicate the level, station, or point, 

 and at the end or right to show the time or day, when this is 

 not the basis of the curve or series. 



Experiment 5. Determining the physical factors of habitats. The in- 

 structor first chooses a series of stations comprising as many different 

 habitats as can be conveniently studied. Each station, and especially 

 those that fall within the same habitat, is located with respect to strik- 

 ing differences of vegetation as well as physical factors. The position 

 of each is fixed permanently by means of a stake. The number of 

 stations is necessarily determined by the size of the class and the number 

 of instruments available. Each observer is furnished with thermometer 

 and psychrometer, and, when the variation in light warrants, with a 

 photometer also. Geotomes, clinometer, compass, barometer, and 

 usually also an anemometer, are carried through upon the preliminary 

 survey by the class and the readings made in common. Returning 

 through the series, an observer is left in each station, and the instructor 

 then places himself at the proper point for signaling. Readings are 

 always made in a fixed sequence. Temperature is taken first, in the 

 order of air, surface, and soil; humidity follows, and finally light. To 

 familiarize the student with methods, and to have a slight check upon 



