BODY AND BRANCH DISEASES AND INJURIES 51 



wide medullary rays, such as oak, beech, ash and maple, are 

 more susceptible to cracking than conifers, although spruce 

 and fir crack frequently. Larger trees, especially those which 

 are isolated and growing on heavy wet soils, are more often 

 affected. 



As the temperature falls below the freezing point, more 

 and more water is withdrawn from the cells of the wood 

 and frozen into ice crystals. This results in the contraction 

 of the wood, the same as if the water were withdrawn by 

 evaporation. The result is a shrinkage tension which, 

 because of the difference in degree of contraction between 

 medullary-ray tissue and lignified woody tissue, exerts a greater 

 pull in the direction of the circumference than in the radial 

 direction. The tension thus developed is considerable before 

 the bark finally gives way and the trunk splits open. The 

 rupture is usually accompanied, because of its suddenness, 

 by a loud report. The splitting is entirely comparable to the 

 checking of timber on drying, except that the water is not 

 lost in evaporation, but remains around and within the elements 

 of tbe wood as ice crystals. With the return of warmer weather, 

 the tissues reabsorb the water lost in freezing and the crack 

 closes. Wound-tissue is formed, but the weakness at this 

 point usually results in a reopening of the crack each winter. 



Repbhences 



Chandler, W. H. The kiUing of plant tissue by low temperatiire. 



Missouri Agr. Exp. Sta. Research Bui. 8 : 141-309. 1913. 

 Zon, R. G. Effects of frost upon forest vegetation. Forest Quarterly 



2 : 14-21. 1903. 

 Stone, G. E. Frost cracks. In Shade trees, characteristics, adapta^ 



tion, diseases and care. Massachusetts Agr. Exp. Sta. Bui. 



170 : 204^205, flgs. 77-78. 1916. 

 Hartig, R. Injuries due to atmospheric influences and fire. The action 



of frost. In Text-book of the diseases of trees, pp. 282-294, flgs. 



157-158. 1894. 



