XXV. 



PLAXT DISEASES. 



TTNTIL within a few years the origin of the dis- 

 ^J eases of plants was considered even more 

 mysterious than that of most diseases of animals. 

 But it is now becoming well known that nearly all 

 plant diseases are produced by certain other plants, 

 usually of minute size and simple structure, which 

 feed upon the living juices of other plants and 

 thus bring about a diseased condition. These 

 peculiar plants which are unable to obtain their 

 nourishment from the soil and air, but must live 

 upon other growing plants, are called parasites. 

 They are nearly all destitute of green color, and 

 belong chiefly to the lower orders of vegetation, 

 known as fungi and bacteria. Not all fungi and 

 bacteria, however, are parasitic and therefore injur- 

 ious, for many live only on matter which is already 

 dead, while a few kinds feed upon both dead and 

 living material. Toadstools and lichens are well 

 known examples of fungi which live mainly on 

 decaying matter. The fungi found on living plants 

 are nearly all of smaller size than these, and can 

 rarely be seen without the aid of the microscope. On 



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