177 



On both July 29 and 31, the high temperature was accom- 

 panied by cloudless skies and a continuous south wind which 

 sometimes reached the velocity of a gale. The relative humidity 

 was very low, probably reaching 30 per cent., although measure- 

 ments were not taken. Under such circumstances no general 

 inconvenience was felt and the skin did not become moist from 

 sensible perspiration. 



In thickets, where the air was not in rapid circulation, even 

 more extreme temperatures were noted. At a height of four 

 feet in the shade of aspen thickets, the temperature rose 2| 

 degrees higher, to io6|°. At the level of the herbaceous layer 

 beneath the thickets, the temperature was 109°, that of the 

 ground debris 107°, and that of the soil two inches deep 85°. 

 In the sun in the aspen thickets, the temperature at four feet 

 was 108°, under the shade of the ground cover of Pteris aguilina 

 112°, at the level of the layer of Vaccinium pennsylvanicum 116°, 

 in the ground debris 125°, and in the soil at a depth of two inches 

 101°. The surface temperature of bare soil with no protective 

 cover reached 140°. 



Such conditions are obviously extreme for the boreal flora of 

 northern Michigan, and some of their effects became visible at 

 once. Leaflets of Rhus glabra wilted, hung vertically downward, 

 and did not fully recover until three days later. Leaves of Acer 

 rubrum became harsh and stiff, bent into irregular position, and 

 exposed the whitened lower surface so freely that the color was 

 visible at a distance of a quarter of a mile. 



In other cases, the effects were not noticed until the following 

 day or later. Leaves of Gaultheria procumbens, Mitchella repens, 

 and Chimaphila umbellata, when growing in the sun, were in 

 many cases killed. Two days later they were brown in color 

 and extremely brittle. Plants of Mitchella repens and Clintonia 

 borealis, which had grown in the shade and had been exposed to 

 the wind by a partial clearing of the surrounding vegetation, 

 showed leaves which were wholly or partly desiccated, even 

 though they had not been directly in the sun. Leaves of Anten- 

 naria neo-dioica, which ordinarily grows in the full sun, became 

 permanently rolled into a half cylinder, exposing the whitened 



