276 



GENERAL PLANT PATHOLOGY 



As an internal determining cause, the formation of enzymes under 

 abnormal conditions must be reckoned as causal, as well, as nutritive 

 disturbances which produce monstrosities and the like. 



Having classified the chief causes of disease, a more detailed descrip- 

 tion of these factors should be put in a form available for student use. 

 Much of the information is scattered, and part of it is buried in foreign 

 botanic and pathologic journals, which can be consulted only in the 

 largest scientific hbraries at home and abroad. 



Fig. 112. — Oyster-shell scale {Lepidosaphes ulmi. After Quaintance, A. 

 Farmers' Bull. 723, Apr. 26, 1916. 



L., U. S. 



The chemic condition of the soil, as a determining cause of disease, 

 may be considered from the standpoint of the normal influence of the 

 important soil ingredients, as contrasted with the absence or deficiency 

 of such elements. Potassium is usually found in young tissues and dis- 

 appears in the older ones. It is associated in some way with the for- 

 mation of carbohydrates in the plant such as starch, sugar and cellulose. 

 The absence of potassium in the soil causes a cessation of growth, the 

 leaves fail to develop the power of forming starch within the greenr 

 coloring bodies, or chloroplasts. A plant which has failed to grow for 

 months will recover in a few days after potassium salts have been added 



