38 BIOLOGICAL LECTURES. 



recent example of such action, a description of the manner 

 in which plants were injured by a late frost in Florida illus- 

 trates the matter quite fully {Botanical Gazette l8, p. 4i7» 1893). 

 In this paper it is noted that all of the young shoots on the 

 limbs of the china tree, Melia Azedarach L., " less than eight 

 feet" from the ground, were destroyed, while those "above 

 twelve feet " were left unharmed. The young shoots on the 

 lower part of shrubs of the prickly ash, Xanthoxylum clava- 

 hercnlis L., were destroyed, while those "above twenty feet" 

 from the ground were uninjured. All the young leaves of the 

 mulberry, Morns alba, within ten feet of the ground were killed. 

 These figures show us the depth of the layer of cold air, with a 

 temperature below the freezing point, as plainly as if we had 

 tested it with a recording thermometer. Very naturally the 

 effects of inversions of temperature are felt only on clear, still 

 nights. A wind would prevent the accumulation of the layer 

 of cold air, and a shield of low-lying clouds would act as a 

 reflector in preventing the escape of the heat from the ground 

 at the usual rate. The device of the orchardist and farmer in 

 building smoky fires at intervals in his crops would both give 

 rise to local currents, which would prevent the accumulation of 

 a cold layer, and would also simulate the presence of clouds ; 

 constituting, in fact, a very effective device for the avoidance 

 of frosts. Coverings, however slight, such as paper or of the 

 thinnest and lightest cloth, would also prevent the greatest loss 

 of heat from the soil underneath the plants thus protected. 



These inversions of temperature occur practically over the 

 entire temperate zone in a manner of importance to the plant, 

 and this phenomenon accounts for the low night temperatures 

 prevalent on all the elevated plains in western North America, 

 in which the effect is emphasized by the comparatively low 

 relative humidity. 



The frosts resulting from this action are not to be confused 

 with the freezing which ensues when great waves of cold air 

 sweep over a section of the country. 



In the summer of 1898 I carried out some ecological obser- 

 vations for the United States Department of Agriculture on 

 the plateau, inclusive of northern Arizona, and lying at an ele- 



