Jan., 1924] PROGRESS OF AGRICULTURAL EXPERIMENTS 13 



Winter Injury of Apple Roots. 



Work on this project has been carried on by G. F. Potter along two lines: 

 First, additional data on injury by low temperature to seedling apples under 

 certain environmental conditions; and, second, microscopical studies of various 

 types of injury to apple roots. 



Observation in the field and, in a limited way, in experiment stations by 

 scientific investigators, has established the fact that the roots of apple trees 

 are more likely to be killed by cold in winter in a dry soil than in one that is 

 partlj^ or nearlj^ saturated with water. Some question has been raised as to 

 whether the greater injury in dry soil is due to the fact that, other things being 

 equal, a drj- soil is colder under given low air temperature than a moist or wet 

 soil. In order to test this hypothesis, thirty-three lots of seedling roots were 

 frozen while embedded in sand containing different amounts of moisture. 

 The roots were placed within cylinders of sand and were so fastened into place 

 that they were all at approximately^ equal distances from the outer edge. By 

 the use of an automatic freezing apparatus, an air temperature of approxi- 

 mately — 8° C. was maintained for nine hours. This period of exposure was 

 sufficient to bring the sand in all the cylinders, wet or dry, to the same tem- 

 perature as that of the air. 



The resulting injury in eleven trials amounted to about 13 per cent less in 

 the dry sand than in either the wet or medium moist sand. 



The injury to the roots frozen in medium moist sand was essentially the same 

 as that in very wet sand, but at the same temperature the dry sand appears to 

 protect the roots. Records of the rate of fall in temperature within the 

 different cylinders indicate that the temperature drops by far the most rap- 

 idly in the dry sand. Therefore, in a cold snap of short duration moist sand- 

 or soil would never get so cold as that with a low moisture content. This 

 would seem to establish the fact that the greater injury to the roots of trees in 

 dry soil is due to the fact that under exposure to a given low air temperature 

 the dry sand would ordinarily become very much colder than the moist sand. 



Twenty freezing tests were also carried on to give additional data on the 

 relation of the rate of freezing to injury to apple seedlings. In this experiment 

 seven lots of roots were frozen in air, the temperature of which was automati- 

 cally allowed to drop about 1.25° C. per hour. In comparison, seven lots of 

 roots were placed in an atmosphere at —8° C. and maintained at this tempera- 

 ture for a long enough period so that the entire root reached the temperature 

 of the air. Under these conditions the live tissues of the root which are near 

 its outer circumference must have dropped to a temperature of —8° almost 

 instantly. A series of roots were placed in air at a temperature at about the 

 freezing point, and the temperature of this air was then lowered to —8° in a 

 period of from twenty minutes to one-half hour. The tissues of these roots 

 would be frozen very much more rapidly than those in the first series, but not 

 so rapidh' as those which were placed instantly under a temperature of —8°. 



The roots subjected to the slow rate of freezing showed injury of less than 

 70 per cent, while those frozen either medium rapidly or very rapidly showed 

 an injury of approximately 94 per cent. It appears from these experiments 

 that a drop in temperature occurring over a period of twenty minutes to one- 



