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 Sderotinia 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. 



