THE STORY OF THE FUNGI i39 



Even though they may not be produced m 

 large numbers, these resting spores are quite 

 sufficient to infect the victims with the coming 

 of the warm weather. When the fungus is 

 mature there is a rapid and abundant produc- 

 tion of the ordinary summer spores, and these 

 will, of course, spread the pest in all directions. 

 Just occasionally the parasitic fungus does not 

 seem to do a great deal of harm to its host. 

 The outgrowths sometimes to be seen in birches, 

 alders, and conifers, commonly known as 

 " witches' brooms," are the outcome of the 

 attacks of a small fungus known as Exoascus 

 turgidus. It should perhaps be pointed out 

 that a very similar result is sometimes trace- 

 able to the energies of certain mites, extremely 

 small creatures related to the spiders. 



Modern research has clearly demonstrated 

 that the outcome of parasitism is not un- 

 commonly a mutually helpful compact. This 

 symbiosis, already instanced in the lichens, 

 is further illustrated by the strange alliance 

 between many fungi and trees. It was at one 

 time thought that the reason for the abundance 

 of fungi in woods was to be found in the fact 

 that so many of these plants thrive on rotting 



