HOW PLANTS BENEFIT AND HARM MANKIND 173 



An examination of specimens shows that the shelf or bracket is 

 in reahty a spore case, which is usually provided with a very con- 

 siderable nimiber of holes, slits, or pores in which the spores are 

 formed. The spores when ripe escape from the under surface of 

 the spore-bearing body through the minute pores. The mycelium 

 is within the tissue of the 

 tree. Remove the bark from 

 any tree infected with bracket 

 fungus, and you will find the 

 silvery threads of the myce- 

 lium sending their greedy 

 hyphte to all parts of the 

 wood adjacent to the spot 

 first attacked by the fungus. 

 This fimgus begins its life by 

 the lodgment of a spore in 

 some part of the tree which 

 has become diseased or broken. 

 Once established on its host, 

 it spreads rapidlj-. There is 

 no remed},' except to kill the 

 tree and burn it, so as to 

 destroy the spores. ]\Iany 

 fine trees, sound except for a 

 slight bruise or other injury, are annually infected and eventually 

 killed. In cities thousands of trees become infected through 

 careless hitching of horses so that the horse may gnaw or crib on the 

 tree, thus exposing a fresh surface on which spores may obtain lodg- 

 ment and grow (see page 142). 



Shelf or bracket fungi on dead tree trunk. 



Suggestions for Field Work. — A field trip to a park or grove near 

 home may show the great destruction of timber by this means. Count the 

 number of perfect trees in a given area. Compare it ^\ith the number of 

 trees attacked by the fungus. Does the fungus appear to be transmitted 

 from one tree to another near at hand? In how many instances can you 

 discover the point where the fungus first attacked the tree? 



Parasitic Fungi. — Of even more importance are the fungi that 

 attack a living host, true parasites. The most important of such 



