CHAPTER XVI 



PLANT DISEASES 



Origin. Plants, like animals, are afflicted with diseases which 

 are caused for the most part by low forms of plant life belong- 

 ing to the great group known as the Thallophytes. Most of 

 these are bacteria or fungi plants without chlorophyll and 

 therefore reduced to the necessity of getting their food ready- 

 made from other organisms. In nourishing themselves they 

 tear down the tissues of the specimen upon which they have 

 fastened, and in due time, if unchecked, may cause its death. 

 Not all, however, thrive upon living things. There are vast 

 numbers that find sustenance in the bodies of dead animals 

 and plants and even in their cast-off parts. Of the latter 

 type are the bacteria of the soil that turn dead vegetation into 

 nitrates and the organisms of decay that resolve dead bodies 

 back into the elements from which they came, thus relieving 

 the soil of forms that would otherwise encumber it. We can 

 easily imagine the confusion that would exist if all the leaves 

 that have fallen in the forest had remained as they were when 

 they fell. The majority of the fungi and bacteria must be 

 classed as helpful species ; it is only when they attack the 

 things we value that they become enemies. As regards the* 

 manner in which they feed, plant pests may be divided into 

 parasites and saprophytes. Parasites feed upon living things, 

 and saprophytes upon dead ones. The organism preyed upon 

 is called the host. In general, parasites are much smaller than 

 saprophytes. The parasites are again divided into the external 

 parasites, which live upon the exterior of the plant and send 

 special organs into its tissues for food; and the internal 



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