124 GROWTH AND WORK OF PLANTS 



MYCORHIZA, SYMBIOSIS. 



205. Mycorhiza. The intimate union of bacteria or fungi 

 with the roots of plants, similar to that described in the preceding 

 paragraphs, gives rise to an interesting and complex structure 

 the work of which is different from that of either part of this struc- 

 ture alone. Such a structure is called a mycorhiza* There are 

 many kinds of mycorhizae, but they all may be arranged into two 

 general kinds. Those where the fungus part is inside of the root 

 are endotrophic mycorhiza, while those in which the fungus part 

 surrounds the root are called ectotrophic mycorhiza. The root 

 tubercles are endotrophic mycorhizae. An example of an ecto- 

 trophic mycorhiza is found in trees of 

 the oak family (oak, beech, hornbean, 

 etc.) and some other trees where the 

 tips of the roots are covered with a 

 dense felt of fungus threads (fig. 93). 

 This occurs in the forest where there 

 is abundant humus (the decaying leaf 

 mold), but is not always present in 

 the same species of trees when growing 

 in fields where humus is absent. The 

 dense mat of fungus threads on the 



Beech root grown in unsterilized outside of the young TOOtS prevents 

 wood humus ; p, strands of fungal 



hypha;, at a associated with humus, the development of root hairs, and 



Magnified several times. (After . 



Frank.) some of the threads penetrate into the 



cells of the roots. The trees, under these circumstances, are 

 dependent on the threads of the fungus for the absorption of water 

 and food solutions from the soil, the threads thus acting as root 

 hairs. There are great advantages to the forest trees in this as- 

 sociation with the fungus mycelium. The fine fungus threads, 

 extending from the mycorhiza, branch and reach out in all direc- 

 tions, penetrating the humus better than root hairs could. They 

 have the power of decomposing certain insoluble nitrogen com- 

 pounds and of passing them over to the tree. They also can 

 change the ammonia compounds, so abundant in humus, into an 

 * From fj-faijs = mold, and ^ifa = root. 



