98 GENERAL CONSIDERATIONS 



plants is the province of vegetable pathology. This 

 is one of the youngest of the great group of biological 

 sciences, since most of our knowledge of plant diseases 

 and their remedies has been gained during the last 

 twenty -five years. It lias, however, the greatest pos- 

 sible practical importance for the farmer, for by the 

 practice of its teachings he can often save his crops 

 from enormous losses. Economic entomology is some- 

 times considered as being distinct from vegetable pa- 

 thology, but it is really only a branch of that impor- 

 tant subject. In a general work of this kind it is 

 impossible to give more than the barest outlines of 

 pathology, and the reader is referred to special works 

 on this subject for a more comprehensive treatment. 



Plant diseases may be divided into three general 

 groups : environmental diseases, functional diseases, 

 and diseases caused by parasites. In the first group 

 are placed all those morbid conditions caused by un- 

 congenial soils, lack or excess of water, or bad climatic 

 or atmospheric conditions. Here, too, would be 

 classed accidental mechanical injuries of all kinds. 

 The only possible remedies for diseases of this class 

 consist in improving the environment or preventing 

 the injuries. If the soil is too wet, it can be drained ; 

 if too dry, it can be irrigated ; if too poor, it can be 

 fertilized. Atmospheric and climatic difficulties are 

 more difficult to avoid, but even here something can 

 be done by planting shelter belts, or giving artificial 

 protection by fires, tents, sheds, etc. The wise farmer 

 will, however, study his surroundings and only plant 

 the kinds of crops to which they are best adapted. 



The term " functional disease" is applied to a class 



