214 



MINNESOTA BOTANICAL STUDIES 



sandhills lie mostly between the isotherms of forty-seven and forty- 

 nine degrees Fahrenheit, and that the region is not therefore the 

 hottest portion of the state. The chart does not, however, indicate 

 in any manner the extreme conditions under which the plants of 

 certain habitats exist during excessively warm days with high 

 desiccating winds that characterize certain portions of the year in 

 the sandhills. Temperature extremes exert a much more powerful 

 influence upon the distribution of life than does the average mean 

 annual temperature. During the vegetative period there occurs in 

 this region a great daily range of temperature both of the air and 

 the soil. After an extremely dry, hot day the temperature may 

 drop to a point that is uncomfortable during the night. Noteworthy 

 diurnal fluctuations in surface soil temperatures have been recorded 

 during these studies. Because of the low specific heat of the dune 

 sand it is found that the surface of the soil over the uplands cools to 

 a low temperature very quickly after sunset and heats rapidly after 

 sunrise. It was found for instance that the maximum surface tem- 

 perature upon the bare sand for a period of twenty-one days during 

 July, 1911, in the central sandhills was 154° Fahrenheit and the 

 minimum for the same period was 76° Fahrenheit. There were a 

 number of days during this period when the temperature maximum 

 approached to within ten degrees of the above maximum. 



. Surface insolation is greatly affected by the aspect and by the 

 presence of even a scanty plant cover. A great difference was also re- 

 corded between the surface temperature of the soil and the tempera- 

 ture at a few inches beneath the surface in the moist sand. The 

 following table exhibits some of the relationships that exist between 

 these particular environmental factors. 



TEMPERATURE OF THE SOIL— JULY 21 

 (Degrees Fahrenheit) 



This table brings out a striking difference between the tempera- 

 tures at the surface and at a depth of six inches even on the hottest 

 days of the summer. The presence of a rather high amount of 

 capillary water may have "a bearing upon this difference. Radia- 

 tion and reflection is great from the more exposed soils. This is 



