242 DISEASE IN PLANTS. 



such as Hornbeam, Beech, Firs, etc., which have 

 been growing in partial shade owing to dense 

 planting, are suddenly isolated by thinning, the 

 impingement of the sun's rays on the south-west 

 side during the hottest part of summer days may 

 kill the cambium, and produce necrosis of the 

 cortical tissues, and such necrotic patches heal 

 very slowly or not at all, because the dead tissues 

 have contracted so tightly on to the wood below 

 that the callus cannot readily creep between. 



Sun-cracks are due to intense insolation on the 

 south side of trees in clear weather in early spring, 

 causing the drying and contraction of the wood 

 and its coverings down that side of the tree : the 

 contracted tissues consequently split, as in the 

 case of frost-cracks, the healing up of which is 

 very similar. 



Dying-back. All that is true of the necrosis of 

 cortical tissues in small patches also applies to 

 cases where the whole of the outer tissues of thin 

 twies and branches die of inanition owing to a 

 premature fall of leaves e.g. after a severe attack 

 of some insect or fungus pest. The consequent 

 arrest of the transpiration current and the proper 

 supply of nutriment to the cambium and cortex 

 explain the phenomena. The younger branches 

 of Coffee trees suffering from severe attacks of 

 leaf-disease are often denuded of leaves and die 

 back from the causes mentioned, the whole of the 

 outer tissues becoming necrotic, and drying up 

 tisrht on to the wood, because other branches with 

 functionally active leaves on them divert the 



