SPECIAL ASPECTS OF NUTRITION OF PLANTS 131 



There are many kinds of mycorhizae, but they all may be ar- 

 ranged into two general kinds, i. Those where the fungus 

 (or the bacteria) is inside of the root. 2. Those in which the 

 fungus part surrounds the root. The root tubercles are exam- 

 ples of the first kind. An example of the second kind 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. 95). This occurs in the forest where 

 there is abundant humus (the decay- 

 ing 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 outside of the young 

 roots prevents the development of root 

 hairs, and some of the threads pene- 

 trate 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 



Beech root grown in unstenlized 



Solutions from the Soil, the threads wo d humus; p, strands of fungal 



^"^.T, a t a associated with humus, 

 several times. (After 



thus acting as root hairs. There are 

 great advantages to the forest trees in 

 this association with the fungus mycelium. The fine fungus 

 threads, extending from the mycorhiza, branch and reach out 

 in all directions, penetrating the humus better than root hairs 

 could. They have the power of decomposing certain insoluble 

 nitrogen compounds and of passing them over to the tree. 

 They also can change the ammonia compounds, so abundant 

 in humus, into an available form for the forest trees. It is 

 supposed that the fungus mycelium obtains some benefit from 

 this association with the roots of trees, but this is not well 

 understood. These mycorhizae are shorter, stouter and also 

 branched more than the normal roots. This difference in 

 form, as well as their more complex structure, renders the name 

 * See paragraph 412. 



