154 NUT GROWING 



of vital energy very much as an epidemic of measles 

 or of smallpox runs out. There are other factors 

 relating to the physiology of resistance that are too 

 technical for inclusion here beyond general statement 

 of the fact. Suffice it to say that an epidemic of 

 potato blight may run out as quickly as an epidemic 

 of scarlet fever, while an epidemic of peach yellows 

 may last as long as an epidemic of yellow fever. 

 Epidemics among plants and animals are caused by 

 microbes, and they include in action the general prin- 

 ciples relating to natural law in the organic world. 



Some of the fungous parasites of plants may re- 

 quire many years before the vital energy of the fam- 

 ily group of these microbes runs out and the epi- 

 demic comes to an end. Insect enemies of plants in 

 cases in which both insects and plants are indig- 

 enous to the locality have so many natural enemies 

 that one may almost depend upon them. The good 

 orchardist, however, is logically one of the natural 

 enemies. It is probably a fact that microbic enemies 

 of plants have quite as many enemies of their own, 

 but there are so many of both that the tree suffers 

 the fate of many another battlefield. 



For many years our butternuts have been grad- 

 ually dying, and in the vicinity of New York very 

 few of the old butternuts are left. A few dead 

 limbs appear and then the trees have a generally un- 

 thrifty look. The cause for this was not known, 

 until recently it was discovered that a fungus, Melan- 

 conium oblongum, was the malefactor, probably an 

 import from abroad. It now threatens our butter- 



