PATHOLOGY 137 



of tlie leaves. Some of the eon.stitueiits of illumiiuLtin^ 

 gases in the air or in the soil are frequent sources of injury 

 and death of plants. 



200. The question of the relative susceptibility of 

 plants to attack by parasites is also comprehended in the 

 term pathology. As yet it is not clear whj^ certain phmts 

 are nearly immune and other plants of the same species 

 are very susceptible to a certain disease. Apparently the 

 difTerence is due parth^ to differences in structure and 

 partly (perhaps chiefiy) to slight differences in the chem- 

 ical composition of the protoplasm or cell sap. The 

 question of induced immunit}^ the effect of changed 

 external conditions upon susceptibility to injury, etc., 

 are very important fields of study that are as yet almost 

 uninvaded. 



201. The study of a plant disease would require then 

 that the student determine the answers to the following 

 questions, and perhaps others as well: (1) What are the 

 pathological symptoms, both structural and plwsiological ? 

 (2) Is the disease caused by a parasite? (3) If not caused 

 b}^ a parasite, what is the cause? (4) If caused by a 

 parasite, what is its life history, particular attention being 

 given to the time and mode of entry into the host, method 

 of propagation, over-wintering, etc.? (5) What are the 

 external conditions, meteorological or cultural, that favor 

 or check the spread of the disease? (6) What differences 

 in susceptibility to the disease are found in different indi- 

 viduals or strains of the host? (7) What is the history of 

 the disease, its distribution, loss caused by it, etc.? (8) 

 In view of the foregoing, how can the disease best be 

 controlled? 



Laboratory Studies. It is iiiipossil)le for a student in this 

 stage of training to undertake laboratory or field studies of any 

 plant diseases. It may not be amiss, however, to have him 



