424 



and it has never been found on any other than these two mushrooms. 

 It is probable that some time in the past it grew on the ground side 

 by side with, and in competition with, the shaggy-mane and inky-cap, 

 but in some way it gained the mastery over its "tender and tasty neigh- 

 bors" and forced them to supply it with food, but by doing so it has 

 made itself entirely dependent upon them. There are a few other 

 species that grow as parasites on other mushrooms, but they are not 

 often found. 



Many of the wood-destroying mushrooms are parasitic for a part 

 of their life and saprophytic during the remainder. That is, they 

 start their life history in a living tree, but after the tree has been killed 

 they continue to live upon it. These fungi are often spoken of as 

 wound parasites, because the only way they can ever get a start in a 

 tree is through a wound, caused by the breaking off of a limb or by 

 other means. No tree would ever become affected with a heart-rot 

 if it were never wounded or if its wounds were always properly taken 

 care of, and one of the main objects of modern tree-surgery is to 

 prevent mushroom spores from getting into the wounds of trees to 

 grow and produce diseases. 



There are also some wood-destroving fungi that live only sap- 

 rophytically. Lcntinus Icpidciis\ for instance, is an umbrella-shaped 

 mushroom which is very fond of growing on railroad ties. It used 

 to cause a great deal of damage until the railroad men learned how 

 to treat their ties with chemical preservatives which make them unfit 

 for mushroom food. The umbrella-shaped mushrooms as a group 

 have always been considered as pure saprophytes. Certainly very 

 many of them are, but it is probable that many more than was form- 

 erly thought are at least partly parasitic. 



Mycorrhisas. — Remarkably interesting structures from an ecolog- 

 ical point of view are the so-called mycorrhizas. A mycorrhiza is a 

 combination of root and fungus, that is, it is a root with a fungus 

 either growing inside of it (endotrophic mycorrhiza) or growing on 

 the outside and entirelv covering it with a coat of mycelium (ecto- 

 trophic mvcorrhiza). Onlv the verv smallest rootlets can f(^rm my- 

 corrhizas, because they are attacked l3y the fungus very soon after the 

 rootlet is produced and the rootlet is always killed within a year, so 

 that the mycorrhizas are never more than a year old. 



In the case of those mycorrhizas in which the fungus is inside of 

 the root, the fungus is usually parasitic on the root for a time and 

 has very much the best of the bargain. Later on, however, the fungus 

 gets tired of the struggle and the root gradually gets the upper hand 

 and finally succeeds in digesting and devouring the fungus. Some of 

 our forest trees, especiallv the maples, have this type of mycorrhiza 



