CHAPTER VI. 



PLANT DISEASES. 

 SECTION XXXV. CAUSES OF PLANT DISEASES. 



By PROF. . H. R. FULTON, 

 Department of Botany, Pennsylvania State College. 



Plants suffer from diseases arising from many 

 causes. Sometimes there is a natural tendency to weak 

 or abnormal development; often the life or usefulness 

 of a plant is threatened by unfavorable surroundings, 

 such as starvation, heat, cold, drought and excessive 

 moisture. In many cases the injury results from the 

 attacks of living animals or plants. 



Fungi. A plant or an animal that gets its food di- 

 rectly from the tissues of another living animal or 

 plant is a parasite; and the animal or plant upon which 

 it feeds is the host. Parasitic plants belong for the 

 most part to the group of fungi; these are plants of 

 low order, which differ from the better known higher 

 plants in size, form, and general life habits. The 

 largest fungi are the toadstools and puffballs, the 

 smallest are the bacteria, and between these extremes 

 are the various molds, mildews, rusts, etc. While 

 many of these can be detected with the unaided eye, 

 the compound microscope is needed for the accurate 

 observation of all but the largest forms. 



Structure of Fungi. Fungi have no roots, stems or 

 leaves. Their bodies usually consist of fine, branch- 

 ing, whitish threads that often increase in length with 

 great rapidity. A mass of these threads is termed the 

 mycelium. The bacteria have bodies consisting of 

 single short cells, or a row of only a few cells, and are 



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