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 their reactions to other parts of the plant. "Internal causes" of disease, therefore, is a phrase expressing our ignorance, but somewhat 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 deficiency 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 true that the presence of such ingredients in excess or in combinations unsuited to the plants also leads to disaster, as also does the presence of minerals or other compounds which poison the root-hairs--_e.g._ products of decomposition, soluble salts of copper and other poisons. That these matters are bound up with the whole question of manuring and of proper soil-analyses will be evident.