CAUSES OF DISEASE. loi 



Finally it remains to say a few words about a 

 numerous class of cases where no external cause 

 of disease can be discovered. It was formerly the 

 custom to group such cases of " Internal Causes " 

 by themselves, but apart from the fact that many of 

 these mysterious diseases have subsequently been 

 shown to be due to the action of external agencies, 

 the whole question of internal causes resolves itself 

 into one of relations between the plant and its 

 surroundings, and it becomes evident that no 

 inherited or internal disease can be regarded as 

 explained until we know the external causes which 

 have so modified the structure and working of the 

 living cells as to make them abnormal in theii 

 reactions to other parts of the plant. " Internal 

 causes " of disease, therefore, is a phrase expressing 

 our ignorance, but somevv'hat more emphatically 

 than usual. If this is clearly understood there 

 seems no reason against its employment for the 

 time being in the artificial scheme of classification 

 we require. With regard to external causes due 

 to the non-living environment, excess or deficiency 

 of materials in the soil, water, or atmosphere plays 

 an important part, and since we may neglect 

 purely aquatic plants it is customary to speak 

 of diseases due to unsuitable soils or to injurious 

 atmospheric influences. For instance, any defici- 

 ency in the supplies of the necessary mineral salts 

 (compounds of calcium, magnesium, potassium 

 with sulphuric, nitric and phosphoric acids, etc.) 

 leads to pathological changes, as also does the lack 

 of the necessary traces of iron. But it is equally 



