Killing of Plant Tissue by Low Temperature 295 



In case of light soils like the "loess" he found that deep planting 

 helps to reduce the danger from root killing. Such soils would of 

 course freeze more deeply and the temperature of the roots would 

 be lower. He had little opportunity to observe directly the differ- 

 ence in hardiness of different stocks, except in one case where Shield 

 Crab was used as stock, three-year nursery trees of Jonathan, Whit- 

 ney, Grimes and Willow came through the winter in good condition, 

 though nursery trees generally were badly injured. In examining 

 injured trees he found that the stock at the union of stock and scion 

 was often killed while immediately adjoining scion tissue was 

 uninjured. He found scion roots to be more resistant than stock 

 roots. This is in accordance with experience here in freezing roots 

 in the laboratory. We have also found that the root system becomes 

 more tender as it gets further from the crown. Emerson and others 

 have observed that roots of trees are killed at a much higher tempera- 

 ture than are other tissues. This has been our experience. Results 

 with freezing roots of orchard trees will be found in Tables 30, 31, 

 32, 33, and 34. 



In case of one-year-old roots of the French crab used as stock 

 by most of the nurserymen, about 5 C.to -8 C. is as low a tempera- 

 ture as they can be depended upon to withstand with no injury. 

 Of 90 Jonathan apple grafts with French crab stock frozen toatempera- 

 ture of -9 C. or lower and then planted in soil in the greenhouse, 

 none lived. Four out of ten lived after being frozen to a tempera- 

 ture of -8 C; three out of ten after a temperature of -7 C; six out 

 of ten after a temperature of -6 C, and practically all grew after a 

 temperature of -5 C. These roots had not been exposed to a high 

 temperature during the winter preceding the freezing, which was done 

 from March 2, to March 18, 1911. 



According to Craig and also Stone, when the roots are frozen, 

 the results do not show to the inexperienced at once in spring. The 

 trees will often bloom and usually the leaves will partially expand 

 before the injury begins to show clearly. If only a part of the root 

 system is killed, only certain connected branches will show the cfTects 

 of the root killing. 



Injury to Apple Wood. Macoun' lists a number of forms of 

 winter injury to wood of fruit trees, especialh' llu' apple, as follows: 



Bark splitting, which he says usually occurs when growth has 

 continued late in autumn and an early summer has prt'\entO(l the 

 soil from freezing. 



'Canada Exp. Farms Kpt. 1907-8, pp llo-Ki. (Ilihl No. OS). 



