Killing of Plant Tissue by Low Temperature 297 



if it were merely a mechanical injury such as tearing loose of the 

 bark. In this case it seems highly probable that the killing resulted 

 from rapid temperature fall. The thawed tissue would fall to the 

 temperature from which it was thawed, very rapidly, and from ex- 

 perience here with rapid freezing (see Table 20) it would seem proba- 

 ble that this tissue was killed by direct freezing the same as buds 

 would be on account of this rapid lowering of the temperature. 



Sun scald is generally thought to result from the effects of the 

 heat gathered by the dark tissue that is in the direct sunlight. Since 

 Macoun reports sun scald as being serious in climates as far north 

 as the Province of Quebec, it would seem doubtful if there would 

 be enough heat gathered on the sunny side of a tree trunk in winter 

 there to raise the tissue to a temperature high enough for growth to 

 take place. From experience here where the rate of temperature 

 fall makes such a large difference in the killing, it seems highly prob- 

 able that, at least in northern sections, sun scald may not result 

 from the tissues starting into growth, but it more likely results from 

 the temperature being raised to near the freezing point on sunny 

 days during cold weather and dropping very rapidly to the tempera- 

 ture of the air as soon as the sun is off that part of the tree trunk. 



As to varieties most hardy in wood under extreme conditions, 

 Macoun^ reports an examination of about 700 varieties in which he 

 found that summer and fall apples were generally more hardy in wood 

 than late winter varieties, probably because their wood reaches a 

 condition of maturity earlier. In his experience some of the most 

 hardy varieties were: Canada Baldwin, Winter Rose, Ontario, 

 Stockton and Mcintosh, all apples of northern origin. Oldenburg 

 was cited by him as being one of the hardiest in wood. 



The group of apples which Hedrick lists as hardy for the coldest 

 part of the state of New York are: of summer apples, Yellow Trans- 

 I)arcnt, Tetofsky, Oldenburg, Red Astrachan, and Lowland Rasp- 

 berry; and of fall and winter apples, Wealthy, Hibernal, and Fa- 

 mcuse groups. Among the varieties that would not withstand the 

 cold in the northern districts were: of summer apples, Early Harvest; 

 and of fall and winter apples, Baldwin, Black Gilliflower, Jonathan, 

 Rome, Winesap, and in the very coldest region, even the Northern 

 Spy group. 



The Killing of Apple Buds. The buds of apples will withstand 

 much lower temperature than will the buds of peaches or of even 

 plums and cherries, and Macoun reports the killing of buds of apples 



'Canada Exp. Frt. Farms. Kpt. 1900. pp. 291-92. (IJlbl. No. 70). 



