INSECTS AND DISEASES 99 



of obscure disorders in which the life processes of the 

 plant are disturbed, and the organs fail to perform 

 their proper functions without any apparent external 

 causes for the abnormal conditions. These diseases 

 are as yet poorly understood, and they have not been 

 sufficiently studied ; but so far as is known they are 

 connected with the improper, either excessive or in- 

 sufficient, secretion of enzymes. These are powerful 

 chemical ferments that are secreted by many cells. It 

 is by their aid that the reserve food stored in so many 

 different parts of plants is made soluble and available 

 for growth and that many of the other life processes 

 are carried forward. The digestive fluids of animals 

 also contain enzymes. When these powerful com- 

 pounds are produced for any reason at the wrong 

 time or in the wrong quantity, serious complications 

 follow. Peach yellows and the mosaic disease of 

 tobacco may be mentioned as well-known examples 

 of functional diseases. So far, unfortunately, no 

 remedies are known for this class of troubles, and 

 they must still be classed as incurable. 



Insect injuries of all kinds are here grouped under 

 the head of diseases caused by parasites, for while 

 some of the losses caused by insects are not, strictly 

 speaking, diseases at all but only mechanical injuries, 

 these grade so imperceptibly into those that cause 

 derangement of function or abnormal growth of 

 tissue that it is impossible to draw a line between 

 them. Parasitic diseases may be classified according 

 to the nature of the damage caused, according to the 

 part of the plant attacked, or, as is more often done, 

 according to the nature of the parasite. Plant parasites 



