ECTOTROPHIC MYCORRHIZAS 



107 



far as external appearance is concerned, and die in the spring. They 

 are therefore annual structures. So far as dicotyledonous trees are 

 concerned these niycorrhizas are pr<)l)a})ly of little importance. 

 The fungi are parasitic on the roots, but ordinarily the percentage 

 of the total ninnber of rootlets of the tree that are affected is so small 

 that the tree is not inconvenienced any more than when a few of its 

 roots are broken off by a burrowing animal or some of its leaves eaten 

 by insects. 



For the other symbionts, however, that is, the fungi, the niycor- 

 rhizas are probably much more important. The roots of trees con- 

 tain a large amount of reserve food at the time when mycorrhizas 

 are being formed and this is undoubtedly made use of by the fungi. 



Fig. 51. — Cross-sections of ectotrophic mycorrhizas. 



right from Picca rubra. 



Left from Carya ovaia; 



Fungi, like all other plants, require a large amount of food for the 

 production of their fruits. Some fungi store up the necessary supply 

 of food, during their vegetative growth, in structures called sclerotia. 

 The mycorrhiza-forming mushrooms, however, depend upon the 

 food in the tree roots for their fruiting activity and the mycorrhizas 

 therefore take the place of sclerotia for these fungi. In the forests 

 of eastern and northeastern United States and of southeastern 

 Canada mushrooms are extremely abundant and ectotrophic mycor- 

 rhizas, which are also abundant, appear to be very important in the 

 nutrition of many of the late summer and autumn mushrooms. In 

 the central part of the Rocky ]Mountain region mycorrhizas do not 

 seem to play so important a part, though in some of the Pacific coast 

 regions they are probably as abundant and important as in the east. 



