186 . NEBRASKA STATE HORTICULTURAL SOCIETY. 



twigs. Though it is less active, this process continues throughout the win- 

 ter. Fortunately, the water loss is greatly diminished by the fall of the 

 leaves,' but the water supply at the roots is also greatly reduced by the 

 freezing of the soil. Where the freezing of the soil is deep and long con- 

 tinued, it often happens that the amount of water lost from the twigs is 

 greater than the amount that the roots can absorb. The result is that 

 the younger twigs shrivel and die just as they would during a summer's 

 drouth. 



The effect of winter dryness naturally differs for different varieties and 

 siiecies of woody plants. Native trees are not "winter-killed" more fre- 

 quently than they die by drouth in the summer, which is a rare occurrence 

 indeed. They have adapted themselves so thoroughly to certain general 

 conditions, by extending the root system, thickening the bark, increasing 

 the number of bud scales, etc., that they pass through the windiest and 

 coldest of winters without drying out. Fruit and shade trees will show 

 this resistance to winter-killing in just as far as they have extended or 

 deepened the root system to get more water, or have protected the 

 branches and shoots against water loss. In connection with the last, the 

 most important factor is the time of ripening of the new growth. This 

 fact is one so well known to practical men since Professor Emerson's in- 

 vestigation of the subject that I need to dwell on it only to point out that 

 the thickening of the bark and the maturing of the wood are both protec- 

 tions against the harmful loss of water during the winter. 



The winter-killing of trees and shrubs has been studied by Nelson in 

 Wyoming (1893), by Goff in Wisconsin (1899), and by Emerson in Ne- 

 braska (1903, 1906). Nelson was the first to point out that dryness was 

 the principal cause of winter-killing. The causes of winter dryness are 

 especially marked in Wyoming, but they are also more marked in Ne- 

 braska than in the states to the north and east. In the first place, the soil 

 as a rule contains less water, and the usual absence of a snow cover per- 

 mits constant evaporation from it. There is more wind and sunshine to 

 increase the loss of water from the twigs and branches. The drying effect 

 of the wind is greatest during our "northers," the very time when the soil 

 water is frozen to the greatest depth. The increasing altitude to the 

 westward results in a decreased air pressure and a corresponding increase 

 in water loss. Goff, studying the effects of the severe freeze of 1899, re- 

 gards cold as the cause of winter-kiiling. The results upon which he 

 bases his conclusion can be more easily explained by the view that winter- 

 killing is due to dryness. Where there was a snow-cover or cover-crop, 

 the damage was slight. This was especially true in all areas where an 

 earlier snow-fall haci drifted and, melting, had increased the amount oi 

 water in the soil. 



Emerson has obtained excellent results by studying the causes which 

 control the ripening of the new growth. His work is especially valuable 

 for its thoroughness and for the preventive measures which he indicates. 

 He has demonstrated the close connection between winter-killing and ma- 



