248 ELEMENTS OF AGRICULTURE 



THE DISEASES OF PLANTS 



By H. H. WHETZEL 

 Professor of Plant Pathology, Cornell University 



Plants, like animals, are subject to many different kinds 

 of diseases. Most of the diseases of plants are caused by 

 insects or by plant organisms, chiefly fungi or bacteria. 

 Some flowering plants, as the dodder, also cause diseases 

 in other plants. 



Bacterial Diseases 



226. Characteristics of Bacteria. Bacteria are the 

 smallest of all known plants. They are to be found almost 

 everywhere on the earth, inside and outside the human 

 body, in milk and water, and even on the dust particles 

 of the air. Like all plants, they grow only where food and 

 moisture are present. Some produce diseases in animals; 

 some cause diseases in plants. By far the most of them 

 are harmless or beneficial. 



Bacteria are among the simplest of plants. They have 

 neither root, leaves, nor flowers, but consist of single cells 

 made up of living protoplasm enclosed within a cell-wall. 

 They are usually spherical, rod-shaped or spiral in form. 

 They are commonly slightly attached to each other in 

 pairs, chains or clusters. Many are surrounded by a muci- 

 laginous substance, which may aid in their distribution. 

 Some are motile, being propelled through the liquid in 

 which they live by long whip-like appendages (flagella). 

 Like all plants, they take their food in solution by diffusion 

 through their cell-walls and protoplasm. They multiply 

 very rapidly, reaching maturity and dividing directly into 



