DISCUSSION OF ECOLOGICAL FACTORS 35 



mometers, and that conditions of humidity at the earth's 

 surface are essentially those one meter above. 



The physical water-content of the soil is least in location 

 I, greatest in II and IV, and intermediate in III. These 

 conditions in a general way hold good throughout the active 

 growing period. Immediately after a heavy precipitation, 

 the water-content of the soil, theoretically at least, is the 

 same for all locations, but the direct exposure of the sun's 

 rays, combined with the greater atmospheric movements in 

 I, produce a much more rapid evaporation than is possible in 

 wooded formations. Accordingly, variations of the physical 

 water-content of the prairie formation are more abrupt than 

 in any other, assuming of course that the soil comj^sition is 

 the same throughout. During certain portions of May and 

 June, 1900, the physical water-content of location I ranged 

 from 3-9 per cent; at other times in June, July, and 

 August, this soil contained 25 per cent and even 35 per cent 

 of water for a short time. Locations II and IV maintained 

 18-26 per cent of water in the soil for the greater portion 

 of the year. Immediately after precipitation, 30-38 per cent 

 of soil water is not uncommon in woodlands; this is reduced 

 to 9-14 -per cent during the drier parts of the year. 



Since the amount of light on a given surface varies as the 

 sine of the angle of elevation, light conditions in the prairie 

 formation are minimum in early spring and autumn, and 

 maximum during midsummer. For each day, there is also 

 a maximum of light (12:00 M.) and two minima (morning 

 and evening). As a result of the foliage, the light relations 

 in locations II, III, and IV are just the reverse of those in 

 location I, viz., maximum light obtaining in the spring and 

 autumn, and minimum light during midsummer. From the 

 above, it follows that the amounts of light in the four loca- 

 tions are nearest equal in early spring, before the foliation 

 of the woody plants, and in autumn after the leaves have 

 fallen. This can readily be seen by a comparison of light 

 ratios given below, which are taken from the ecological fac- 



