CAUSES OF DISEASE. 423 



constitutionally weak. This, according to Dr. Van 

 Mons, is owing to a diseased state of the parent at 

 the formation of the seed. 



2d. The attacks of insects, induced hi/ a state of 

 iveahiess. Many insects attack more readily a weak 

 tree, and, feeding upon its vitiated sap, decrease its 

 strength, and cause ultimately its death. 



3d, The influence of a poor stock on which the tree 

 is grafted. As these stocks are seedling plants, they 

 of course vary as much in constitution as the culti- 

 vated sorts. If the stock be naturally weak, the 

 scion inserted therein partakes of this weakness, and 

 seldom attains full size ; while, if the stock be con- 

 stitutionally strong and robust, and the scion insert- 

 ed therein weak, the vigor of the former will over- 

 come the weakness of the latter. This accounts 

 not only for the varying strength, but for the differ- 

 ence in the quality of trees of the same variety, and 

 under the same conditions of culture. 



4th. The use of improper scions. Fruitfulness is 

 often delayed by using scions from young undevel- 

 oped trees ; or weakness may be produced by the 

 use of those from an old and decrepid tree. If pos- 

 sible, the scions should be taken from a tree which 

 has just commenced to bear, and is in the prime of 

 life. 



5th. Bad soil. If the soil is cold, however great 

 may be the constitutional vigor of the tree, canker, 

 or some other disease will, sooner or later, appear. 



