458 NITROGEN IMETABOLISM 



Ectrotropic mycorrhizas are found on many forest tree species such as 

 beeches, oaks, hickories and many conifers. They are particularly abundant 

 on trees growing in soils rich in humus. The fungal associates in forest 

 tree mycorrhizas are mostly members of the group of Basidiomycetes, many 

 of them apparently being common woodland species of mushrooms. 



Endotropic mycorrhizas are found on many species of the orchid, heath, 

 and gentian families, and also on some trees, such as the red maple and walnut. 

 The fungous associates in such mycorrhizas are apparently mostly microscopic 

 molds. 



Diverse opinions have been advanced regarding the significance of these 

 root-fungus associations. Undoubtedly the mycorrhizal fungi associated 

 with chlorophyllous plants obtain most or all of their carbon-containing com- 

 pounds from the species on which they grow. Hence some authorities con- 

 sider them to be no more than root parasites. At least some mycorrhizas, 

 however, are apparently beneficial to their associates. This seems to be 

 especially true of the mycorrhizas of conifers, particularly when they are 

 growing in distinctly acid soils which are low in nitrates. There is evidence 

 that on such soils mycorrhizas aid in making available to their host plants 

 some of the nitrogen of the complex organic compounds found in the humus 

 (Rayner, 1926, 1927)- 



Discussion Questions 



1. List the different kinds of chemical combination in which nitrogen is present 



in green plants. 



2. List and discuss the various ways in which the "C/N ratio" of plants can 



be increased ; ways in which it can be decreased. 



3. Why does not the continuous occupancy of an uncultivated area by native 



plants eventually exhaust all the nitrogen in the soil? 



4. Corn (maize) was planted upon a field that was in alfalfa the preceding 



year. Trace the origin of the various kinds of nitrogen compounds which 

 would be available to the corn plants. 



5. What are some of the important inter-relationships between respiration and 



the nitrogen metabolism of plants? 



6. How could you demonstrate convincingly that green plants cannot use the 



nitrogen of the atmosphere? 



7. It is often possible to induce the resumption of flowering and fruit produc- 



tion in old apple trees that have ceased bearing by heavy pruning coupled 

 with the application of nitrogenous fertilizers to the soil. Explain. 



8. In general would you expect soil nitrogen conditions which are favorable 



to the development of good crops of lettuce or celery to be equally favor- 

 able for the development of Irish potatoes? Explain. 



9. In general would you expect the "C/N ratio" of a tomato plant to be higher 



if grown at 15° C. or if grown at 25° C. Explain. 

 10. Mature fruit trees often flower unusually heavily the spring following a 

 dry year. Explain. 



