of plants, in which he showed that the northernmost Hmits of 

 southern species might be expected on hill tops, other things 

 being equal, and the southernmost limits of northern species in 

 the valleys. By inversion of temperature is meant the cooling 

 at night of the lower layers of air by conduction to the soil, so 

 that they reach a temperature some degrees below that of the 

 upper strata. In a hilly or broken country this cold air settles 

 by its greater weight into the valleys and the hill tops are covered 

 with a constant supply of warmer air, with the result that their 

 average temperature is higher and extremes less marked than in 

 the valleys below. 



There are a number of facts connected with plant distribution 

 which help substantiate this claim, and it would seem that inver- 

 sions of temperature are of considerable importance in such a 

 broken country as the Ozark region of Southern Illinois. Bald 

 Knob, in Union County, is a somewhat conical hill, rising over 

 150 meters above the surrounding valleys, and nearly 1 00 meters 

 above the general level of the country. Its total height is just 

 300 meters. The farmer who cultivates what little arable land 

 there is on its summit is able to market his tomatoes some days 

 before his neighbors and never loses his crop by late frosts in 

 the spring. The first frosts in autumn are also two to three 

 weeks later than in the surrounding country. The general 

 effect of this condition on the native flora is seen all through the 

 southern end of the state, where the southern flora, two or three 

 hundred species of which occur, is found almost exclusively 

 either on the highest and driest uplands or in the swamps. The 

 former situation is probably due to a temperature relation, the 

 plants finding there a temperature more nearly like that farther 

 south, which in a measure compensates for the unfavorable soil. 

 The latter situation, in the swamps, is due to a water relation, which 

 being near the optimum, permits the growth of southern species 

 in spite of the lower temperatures. 



The distribution of the yellow pine, Pums ecliinata, which here 

 reaches its northernmost limit in the Mississippi Valley, may be 

 taken as an example. It is confined to a few steep-sided narrow- 

 topped ridges from 80 to 120 meters high, where the influence 



