PATHOLOGY 135 
In all these cases it is not the parasite but its effect upon 
the host that should be the subject of pathological in- 
vestigation by the botanist. It must be remembered 
that merely to learn the name of the organism causing the 
pathological change in a plant is not to study pathology. 
It is the investigation of the actual physiological and 
structural changes in the diseased tissues that deserves 
that name. . 
197. By far the greater number of plant diseases 
hitherto investigated are those caused by parasitic plants 
(bacteria, fungi and flowering plants). As in the case of 
injury by animal parasites the effects are very varied. 
Thus with some parasites the injury consists of perhaps 
hardly more than the withdrawal of food stuffs or water 
from the tissues of the host. Usually, however, the case 
is not so simple. There is almost always some mechanical 
. disturbance as, for example, the destruction of the middle 
lamella to permit the intercellular growth of a fungus 
hypha or perhaps the actual crushing of some of the cells 
of the host by the roots of some of the parasitic flowering 
plants. A few parasites kill the cells some distance in 
advance of their progress by the secretion of poisons of 
various kinds (as is the case with Sclerotinia libertiana), 
feeding then upon the more or less disorganized remains 
of the dead cells. In other cases, however, the parasite 
does not kill the host cells outright but sends little 
branches (haustoria) into them through which the food 
matters are gradually absorbed, the death of the cell 
perhaps being delayed for a long period during which it is 
constantly furnishing food to its parasite. Sometimes 
the diseased tissues become enlarged and richly stored 
with food (various fungus galls, e.g. peach leaf curl due to 
Exoascus deformans) which may then be used by the 
fungus. 
