Microorganisms and Plant Life 187 



are more readily recovered from roots than from stems and 

 leaves. Little significance can be attached to their presence, 

 however, for they occur in small numbers, and their pres- 

 ence seems to be unrelated to plant vigor or survival. 



MYCORRHIZAE 



Mycotrophy in plants can be interpreted broadly as the 

 associative development of fungi and plants, in which the 

 fungi penetrate the plant tissue (16, 20, 22, 33). It is a 

 very general phenomenon in nature and has been reported 

 for all of the major groups of plants, including the algae, 

 mosses and liverworts, ferns, gymnosperms and angio- 

 sperms. Particular attention has been devoted to the asso- 

 ciations of fungi with orchids, forest trees, and ericaceous 

 plants. There are two major types of fungus development: 

 the ectotrophic, in which the fungus makes profuse growth 

 about the root surface and develops between the cells of 

 the epidermis; and the endotrophic, where there is little 

 external mycelium but the fungus penetrates deeply into 

 the tissues and invades the cells. There is also a type re- 

 ferred to as ectendotrophic, where the fungus development 

 is intermediate between the two. The significance of de- 

 velopment of the fungus associates has been variously inter- 

 preted, and it is probable that with different plants the 

 effects of the fungi differ. In plants that have limited, if 

 any, root systems, the fungus functions as root hairs. Kelley 

 (20) has said: "It is unknown whether plants in nature 

 have root-hairs or mycorrhizae, or neither; but there is 

 enough evidence at hand to indicate that mycorrhizae pre- 

 dominate over root-hairs in the majority of cases." In 

 chlorophyll-free plants, the fungi may provide the plant 

 with organic materials. With pines and certain other trees, 

 the fungi cause the development of an abundance of short 

 roots and thus provide greater absorbing surface (15, 16, 

 17). Furthermore, minerals become absorbed through the 

 fungus mycelium which serves as root hairs (23). 



