191 1.] The Safe Working of Steam Engines, etc. 315 



Europe as a dangerous parasite, but in the Mississippi Valley 

 apple-growing region of the United States it is a constant 

 source of danger as a true parasite. The disease is usually 

 found upon the larger branches, and, the mycelium being 

 perennial in the tissues, a wound once formed continues to 

 increase in size, until eventually the branch breaks away at 

 the wounded part, and in many instances the entire tree is 

 killed outright. 



The first indication of infection is a brown and more or 

 less sunken appearance of the bark in patches, which are 

 usually scattered over a considerable area. These patches 

 eventually become dry, cracked, and blackened, and the black 

 fruit bodies of the fungus appear, at first as small globose 

 bodies, which gradually expand into shallow saucer-shaped 

 structures with a thick raised margin. Eventually the dry, 

 dead bark falls away, but the fruit bodies of the fungus 

 remain firmly attached to the wood. The mycelium present 

 in the wood gradually encroaches on the surrounding healthy 

 parts from year to year, hence the canker continues to increase 

 in area until the branch is completely girdled and consequently 

 dies. 



The fungus is a wound parasite, and as spores are produced 

 by the fruit-bodies in great abundance, the chances of infec- 

 tion through wounds caused by atmospheric agencies, birds, 

 insects, &c, are unlimited. 



Owing to the fact that this fungus has proved so destructive 

 to apple trees in the United States, it is imperative that it 

 should be removed and destroyed when met with in this 

 country, as judging from analogy a fungus that has hitherto 

 been known only as a harmless saprophyte, may at any 

 moment become a dangerous parasite. 



There is reason to believe that in agricultural districts con- 

 siderable ignorance prevails with regard to the precautions 

 which are necessary for the safe working 

 Precautions for the Q f steam engines and boilers. 

 Stea^^ngines^ Every year serious explosions from 



and Boilers. these engines and boilers occur, 



through which there is loss of life or 

 njury to persons, and frequently damage to property. 



