32 DISEASES OF CULTIVATED PLANTS 



is experienced. On the other hand, if thawing takes place 

 rapidly, the liberated water escapes into the intercellular 

 spaces instead of being absorbed by the cells, and the death 

 of the organ results. From the above account it follows that 

 when plants have been frozen late in the spring they should 

 be protected by a covering of some kind from the sun's rays, 

 so that they may thaw gradually and slowly. When the 

 covering method is impracticable, a thorough syringing with 

 cold water is often effective in preventing rapid thawing. 



When actively growing leaves are frozen late in the spring, 

 the epidermis of the under surface is often completely torn 

 away from the rest of the leaf, and hangs like a blister. I have 

 seen a whole field of cabbages affected in this manner, with 

 the large lower leaves hanging limp on the ground, and the 

 lower epidermis raised in large blisters. The young leaves of 

 Aucuba and other evergreens also suffer in a similar manner. 



Frost cracks. These are longitudinal cracks of variable 

 length in the bark and wood of tree trunks, and are generally 

 the result of a sudden and great reduction of temperature. 

 The cracking is due to the abstraction of water from the cell- 

 walls, which freezes in the cavities of the cells. Owing to 

 this loss of water from the walls the cells shrink and a crack 

 is the result. When the temperature rises the cells expand 

 by again absorbing water, and the crack closes up more or 

 less, and is soon protected by a growth of callus along the 

 two edges of the wound, which projects more or less above 

 the level of the adjoining bark, and is known as a ' frost-rib/ 



In some instances a frost-crack that has healed up is again 

 opened by frost, when the same healing process is repeated. 

 If this rupturing occurs repeatedly a very prominent frost-rib 

 results, which may finally remain permanently open or closed 

 depending on weather conditions. 



Catalpa bignonioides, an ornamental tree, is a native of 

 the southern United States, and is often injured by frost 

 when planted in the New England States, and in this country. 

 A very fine specimen, about sixty years old, recently succumbed 

 from frost in Kew Gardens. The tree had two tiers of 

 principal branches, an upper tier of four nearly horizontal 

 branches and lower down on the trunk three larger ones. 

 When the upper branches were about forty years old, each 

 one was injured by a severe frost, which produced a frost 

 wound throughout its entire length. These wounds never 



