BACTERIA OF THE SOIL 259 



practiced. When once the bacteria are thoroughly inocu- 

 lated through the soil, that is, after there have been one or 

 more successful crops of legumes grown on a particular 

 area, the organisms will persist for a considerable period 

 of time even though the particular legume is not present. 



Nitrogen Fixation by Symbiotic Fungi. — Many kinds of 

 higher plants are found to have growing in or upon their 

 roots species of fungi apparently intimately connected with 

 the metabolism, life history and successful development of 

 the plant. The plants which show this characteristic are in 

 many cases inhabitants of swamps and bogs, others are the 

 so-called epiphytes or air plants. In addition, however, 

 many species are known to possess these fungi upon their 

 roots, though they grow under normal soil conditions. 

 Fungi growing in or upon plant roots are generally termed 

 mycorrhiza. In some species the fungi occur inside the root, 

 frequently giving rise to swellings, nodules, tuber forma- 

 tion, etc. In other cases the fungi grow over the surface of 

 the root, penetration, if any, being relatively shallow. The 

 former are termed endotrophic and the latter ectotrophic. 



Among the endotrophic mycorrhizas are some which grow 

 in the roots of orchids. All members of this great plant 

 family (Orchidacew) show a certain zone in the roots just 

 inside the epidermis given over to hyphse of a fungus. It is 

 found that seeds will die soon after germination, if this 

 fungus is not present and functioning. The roots of the 

 Eussian olive tree, furthermore, and its close relatives, such 

 as the shepherd berry, have nodules on their roots much 

 resembling the nodules of legumes, but harboring an endo- 

 trophic fungus. The same is true of the roots of the alder 

 and the New Jersey tea of our prairies. Apparently these 

 nodules are normal to the plant and the organisms growing 

 in them are in some respects beneficial. In certain species 

 of flowering plants which have lost the power to produce 

 ehlorophyl, such as the Dutchman's pipe, apparently the 



