PATHOLOGY 137 
of the leaves. Some of the constituents of illuminating 
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 itis not clear why certain plants 
are nearly immune and other plants of the same species 
are very susceptible to a certain disease. Apparently the 
difference is due partly to differences in structure and 
partly (perhaps chiefly) to slight differences in the chem- 
ical composition of the protoplasm or cell sap. The 
question of induced immunity, 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 physiological? 
(2) Is the disease caused by a parasite? (3) If not caused 
by 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, etce.? (8) 
In view of the foregoing, how can the disease best be 
controlled? 
Laboratory Studies. It is impossible 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 
