244 



Plant Physiology 



of mycorhiza occurs also in several orchids and in a few 

 other plants. Many common forest trees, such as beech 

 and pine, likewise exhibit mycorhiza. In the latter case 

 the fungus invests the root with a mycelial weft, the threads 

 merely coming in close contact with the cells (ectophytic). 

 These fungi are believed to be not only of importance to 

 the tree as absorbing organs for water and nutrient salts, 

 but possibly in the fixation of nitrogen. Nevertheless, 

 this point has not been established. 



137. General sources of supply of nitrogen. From 

 the data presented it is apparent that the nitrogen problem 

 in plant production is one of peculiar interest and diver- 

 sity. Commercial sources of nitrogen for plant production 

 may be natural supplies of nitrates, compounds of am- 

 monium (chiefly the sulface, as a by-product of coke and 

 gas making), waste and prepared animal products, and 

 green-manure crops (especially legumes). In addition, 

 good conditions for fixation by bacteria and fungi and 



