CHAPTER XVII 

 PLANT DISEASES 



PLANT pathology is a comparatively new science. It is 

 only within recent years that it has been recognized that 

 plants are subject to diseases just as are animals, and that 

 these diseases in turn are proper topics for scientific study. 

 It has been vaguely recognized for a long time that certain 

 factors have retarded the activity of the plant, causing loss 

 of crops and in many cases entire destruction to the plants 

 involved. A scientific study of the phenomena was not under- 

 taken, however, until well along toward the close of the 

 nineteenth century. At present, every well organized college 

 of agriculture has a department of plant pathology; and 

 research in this field is adding immensely to man's knowledge 

 of the subject and consequently to his power of control. 



356. Causes of plant diseases. The cause of most plant 

 diseases is some invading organism belonging to the lower 

 forms of life, that is, to the bacteria and other fungi. It may 

 be said in passing, that a fungus is itself a plant that possesses 

 no green coloring matter. On that account, it cannot manu- 

 facture its own food and it lacks in structure many of the 

 characteristics of the higher plants. Organisms of this sort 

 invade the tissue of the higher forms, sapping the nourish- 

 ment from them, and deranging their functions in such ways 

 as to produce symptoms of disease. 



357. Fungi. As already stated, fungi are plants; they 

 are not like those with which we are acquainted, in that they 

 are very much simpler in their structure. They have no roots, 

 stem, leaves, flowers, or seeds, as these terms are defined in 



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