ROOT DISEASES AND INJURIES 83 



sists in the development of myeorhizas. These structures 

 probably in no manner interfere sufficiently with the growth of 

 the roots to cause any damage. Trees with myeorhizas on the 

 roots cannot be distinguished from those without by any above- 

 ground symptoms. A short account of them is, however, of 

 interest since the structures are now generally considered to 

 represent a diseased condition of the roots and not a true type 

 of symbiosis or mutual-advantage relation, as was previously 

 believed by many. 



The term mycorhiza is used to signify the infected root and 

 the mycelium of the fungus, as an association of two distinct 

 but physiologically interdependent tissues. This usage is thus 

 similar to the use of the term lichen, the formation of which 

 requires the association of certain algse and fungi to form the 

 characteristic structures known as lichens. Myeorhizas are 

 of two t\'pes : ectotrophic, when the mycelium forms a mantle 

 or sheath around the root-tip and penetrates the tissues by inter- 

 cellular (existing between the cells) threads of the mycelium; 

 and endotr(y)hic, when the mycelium is within the tissue and is 

 largely intracellular (within the cells). Recent investigations 

 have shown that hickory, oak, basswood, birch, larch, poplar 

 and beech commonly show ectotrophic myeorhizas, and species 

 of maple, butternut and horse-chestnut ha^e endotrophic forms. 

 Elm and species of willows were found not to have myeorhizas. 

 This list is by no means complete, but represents the species 

 reported m the reference given below. With the exception of 

 this work, little is reported in American literature on myeorhi- 

 zas. European literature on the subject, however, is abundant. 



Both tj'pes of myeorhizas are annual. The mycelium of the 

 species, of fungi capable of forming myeorhizas penetrates the 

 young root-tips in the summer. The association with the root- 

 tissues is rapidly formed and food materials are obtained by the 

 fungus. Later, the fungus may produce a growth of mycelium 

 in the soil in the autumn, in which case fruiting-bodies are 



