DISCUSSION OF ECOLOGICAL FACTORS 33 



carefully weighed, the contents dried in an oven, and again 

 weighed. The loss in weight represents the physical water- 

 content. 



To obtain light factors in the four locations, strips of 

 "Solio" paper were ruled lightly with a violet lead and ex- 

 posed to the sun's rays until the lines entirely disappeared, 

 the time required in each instance being noted. Observations 

 were made on clear days between 12:00 and 2:30 p.m. In 

 determining light factors, there are many difficulties to be 

 overcome before even fairly accurate results may be expected. 

 The angle of inclination of the sensitized paper to the sun's 

 rays is not always the same, the sky is not uniformly clear 

 even though cloudless, and, disregarding deflection, reflec- 

 tion, and absorption, the amount of light received on a given 

 surface varies as the sine of the angle of elevation. Accord- 

 ingly, paper exposed in woodland or thicket for a given 

 period will receive much more than one-half of the total 

 amount of light during the first half of the exposure. As 

 stated heretofore, these difficulties are already being antici- 

 pated. 



A careful examination of the ecological factors found in 

 another part will reveal many interesting relations, the more 

 important of which are here noted. Atmospheric and soil 

 temperatures, humidity, physical water-content, light ratios, 

 and, in fact, most other ecological factors are minimum in 

 the four locations during the spring and autumn months 

 and become maximum only during the summer months. The 

 air temperature of the prairie formation is from .5°-10° 

 higher than corresponding temperatures in deep woodlands, 

 from .2°-3.5° higher than temperatures in open woodlands, 

 and from .5°-10° higher than corresponding temperatures 

 in dense thickets. If for the time we consider prairie con- 

 ditions normal, we shall conclude that these differences are 

 due to the controlling woody plants in locations II, III, and 

 IV, since where the trees are less controlling, as in III, 

 temperature differences are accordingly reduced. Tem- 

 3 



