425 



as also have many other plants, such as the orchids. Indeed, many of 

 the orchids have become entirely dependent upon the mycorrhizal 

 fungi. In some orchids the seeds will not germinate except in the 

 presence of the proper fungus, while in others the seeds will germinate 

 but the seedlings never grow beyond a certain stage unless they be- 

 come associated with the fungus. The kinds of fungi that cause this 

 type of mycorrhiza is in most cases not known, because their fruit 

 bodies have not been seen. Only a few of those associated with the 

 orchids have been identified and in no case were they found to be 

 mushrooms. 



The other tvpe of mycorrhiza (ectotrophic), in which the mycelium 

 of the fungus is mostly on the outside of the root, is found on many 

 of our forest trees, such as ash, hickory, beech, linden, etc., as well as 

 on many shrubs and herbaceous plants. These mycorrhizas were until 

 recently not very well understood. They were first described about 

 thirty years ago and for a long time after that it was thought that 

 they were of great importance to the plants on which they occur, in 

 that they helped to absorb from the soil certain materials which the 

 plants would otherwise be unable to get. It is now believed, how- 

 ever, that they are of no benefit in any way to the plants on which 

 they occur. The fungus is merely a parasite on the root. Ordinarily 

 these parasites are cfuite harmless to a tree because only a small per- 

 centage of its roots are affected, and it really does not sufi^er any 

 more than it would if we were to cut ofi a few of its roots or pull 

 off a few of its leaves. 



It seems likely, therefore, that these mycorrhizas are of much more 

 importance in the life of the fungi which cause them than they are 

 to the plants on which they are found. It is now known that most 

 if not all of the ectotrophic mycorrhizas are caused by mushrooms . It 

 is probable that very many of our late summer and fall mushroomS: 

 especially those which grow in the woods, are capable of forming 

 mycorrhizas on the roots of trees. The fruit bodies of these mush- 

 rooms are usually produced soon after the mycelium becomes attached 

 to the roots, and it is possible that the fungi have great need of the 

 particular kind of food that they get from the roots for the develop- 

 ment of the fruit bodies. 



Any one can observe these mycorrhizas by digging up some of the 

 roots of trees growing in the woods in the fall of the year. They 

 appear as little clusters of short, stubby root-branches, usually white 

 or whitish, but sometimes colored — brown, yellow, red, etc. 



Auiinal Relations. — We have alreadv spoken of the ways in which 

 animals aid in the scattering of mushroom spores. There are a num- 

 ber of other ways in which animals affect the life of mushrooms. Chief 



