PART II 

 GENERAL PLANT PATHOLOGY 



CHAPTER XXIII 

 GENERAL CONSIDERATION OF PLANT DISEASES 



The student who would become acquainted with the general 

 pathology of plants must have some previous knowledge of other sub- 

 jects, especially those which are concerned with the life of the plant. 

 To appreciate diseased conditions the normal state of the plant must 

 be understood. A study of phytopathology, which as a department of 

 scientific inquiry concerns itself with plant diseases, therefore, presup- 

 poses that the would-be phytopathologist is acquainted with plant 

 morphology, systematic botany (fungi and flowering plants) histology, 

 cytology, embryology, genetics, physiology, with bacteriology, zoology 

 (especially entomology) chemistry and physics,' as well as meteorology. 

 Plant morphology deals with the general form and gross structure of 

 plant parts, such as roots, stems, leaves, flowers, fruits and seeds. The 

 student should know the common fungi (see part I), the technique of 

 their study (see part IV), as well as the flowering plants, which act as 

 hosts to the bacteria and fungi causing disease. Histology is concerned 

 with the microscopic details of plants, while cytology treats of cell 

 structure and organization. Embryology, as a distinct subject of in- 

 quiry, embraces a study of their productive cells and organs. Genetics 

 is a new branch of inquiry. As Walter tersely put it, "The study of the 

 origin of the individual, which has grown out of the more general 

 consideration of the origin of species, forms the subject matter of 

 heredity, or, to use the more definitive word of Bateson, of genetics." 

 The functions of a plant are considered when we study physiology and 

 the chief divisions of that subject treat of the nutrition, growth and 



•Along these lines see suggestive papers by Ernest Shaw Reynolds: Plant 

 Pathology in its Relations to other Sciences. Science, new ser., xxvii: 937-940; 

 June 19, 1908. 



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