AIR DRAINAGE AS AFFECTING ACCLIMATIZATION. 27 



28, and on the 26th, 26. Immediately after sunrise the tem- 

 perature rose at once to above freezing. This lowering of tem- 

 perature was not due to the down-flow of cold air from above but 

 solely to radiation from the soil and other objects. This was dem- 

 onstrated by the fact that a lath roof which could not have pre- 

 vented the inflow of cold air from above, for the spaces between 

 the laths were the width of the laths themselves, prevented 

 radiation to such a degree that pineapples under this roof suf- 

 fered no injury, while where the roof was lacking, owing to 

 previous destruction by wind, the plants were badly injured, 

 although no lateral barrier was between the plants. 



A few days after the freeze I drove all around the country 

 about Miami making observations upon the effects of the cold. 

 It was at once apparent that the layer of air causing the dam- 

 age was not deep. In few places did any injury appear more 

 than four feet above the ground, while in many spots the injury 

 was confined to within a foot or two. At the Subtropical Gar- 

 den a number of two-year-old mango and avocado trees were 

 killed. For several days, in one case for several weeks, the 

 tops remained green, then suddenly died. I found that for a 

 distance of six inches to a foot from the ground the bark and 

 young wood had been killed. The same observation was made 

 on a number of tamarind, india rubber and other trees set out 

 along the avenue for ornamental purposes. 



In several places the injury to the foliage of tender plants 

 was noticeable from the ground up to a certain level, the line of 

 demarcation between killed and uninjured foliage being very 

 distinct and perfectly level. In one place, especially, where a 

 grove of avocado trees stood in a kind of basin about one hun- 

 dred yards across, the injury was not apparent to the trees at the 

 margin but reached higher and higher the further one advanced 

 into the center of the basin, while at the other side the injury 

 again became less and less. Sighting across it was clear that a 

 lake of cold air, level at the top, had occupied this basin. 



In marked contrast to such places as this was the absolute 

 lack of injury where the slope of the surface was such as to 

 permit the cold air to flow off. Thus at Cocoanut Grove, about 

 four miles south of Miami, the land near the bay is sloping 



