No. 387.] PLANT MORPHOLOGY AND PHYSIOLOGY. 211 
Pror. D. T. Mac DouGaL: Znfluence of Inversions of Temperature 
and Vertical Air Currents upon the Distribution of Plants. — The soil 
and the air resting upon it receive the same amount of heat during 
the day, but at sunset the temperature of the earth is slightly higher 
than that of the air. At this time both begin to lose heat, but the 
soil cools much more rapidly than the air. The air is a poorer con- 
ductor, than the soil, and hence the layers of air resting immediately 
upon it are cooled by radiation and conduction to the cold surface 
to a temperature far below that of the body of the air a few meters 
above. The consequences of this inversion are to be seen in the 
effects of late spring frosts, when the lower branches of a tree or 
shrub may be injured while the upper ones will be unharmed. 
This nocturnal inversion of temperature occurs over almost all 
land areas, but is most marked in regions of low relative humidity. 
In North America it is most pronounced on the elevated plains, where 
it is a distinct but heretofore unrecognized factor in determining the 
boundaries of life zones. 
In broken countries the cooling of the surface layers of air results 
in its contraction and increase in weight, and, as a consequence, the 
cold air thus formed on elevated mesas, ridges, and hilltops flows 
down the slopes into the depressions and valleys, filling the latter with 
a deep layer of cooled air while a constant supply of warm air settles 
down on the highlands. As a result of this action, the hills and 
lower mountain ridges have a much more equable temperature than 
the valleys and cafions. ‘Thermographic records obtained at Flag- 
staff, Arizona (in a valley 6862 feet above sea level), and on Observ- 
atory hill (on the west side of the valley at an elevation of 7162 
feet), in June and July, 1898, show that the minimum temperature of 
the valley was 15° to 27° F. lower than that of the hill at the same 
time. 
If the slopes of the hill or mountain are several thousand feet in 
vertical extension, the descending current may sweep down so rapidly 
as to actually increase in temperature and reach the valley below as 
a warm wind. Regular currents of this sort are rare. The “ Chinook ” 
or “ Foehn ” owes its warmth to this cause. 
Again, the upward movement of the air, under the influence of the 
sun’s rays during the day, results in an expansion and absorption of 
some of the heat, so that these currents reach the highlands at a 
lower temperature than the air resting on such areas, and tend to an 
equalization of the temperature. At the stations mentioned, the 
maximum temperature of the hill was always 4° to 6° F. below that 
