BIOLOGICAL FACTORS 195 



and penetrate the spaces between cells (ectotrophic mycorhiza), 

 or the fungus may occur within the cortical cells of the root only 

 (endotrophic mycorhiza). Such root-fungus relationships are far 

 more common than was once supposed. It is known that they 

 occur on most forest trees and shrubs and that many herbaceous 

 plants may have them. They form during periods favorable to 

 root growth and are practically restricted to the young roots in 

 the surface strata of the soil. 



Whether mycorhizas represent a mutualistic relationship or 

 merely parasitism on the part of the fungus has been strongly 

 argued by numerous investigators. The conflicting evidence makes 

 interesting, if somewhat confusing, reading. However, the evi- 

 dence that mycorhiza must be present for the successful growth 

 of many species, particularly forest species, is sufficient to suggest 

 that the mycorhizal condition is desirable under most situations 

 even though the reasons are not too obvious. 



Pot cultures of certain tree seedlings in poor soil have been un- 

 satisfactory until inoculated with mycorhizal fungi. On a larger 

 scale, unsuccessful forest nurseries on prairie soil or long deforested 

 agricultural soil have been saved by bringing in small amounts of 

 forest soil, which started the formation of mycorhiza. Tree seed- 

 lings transplanted without mycorhiza to treeless areas have been 

 saved from gradual death by the application of small amounts of 

 soil containing mycorhizal fungi. 



Several members of the heath family (azalea, rhododendron, 

 blueberry) are dependent upon the presence of mycorhiza that 

 cannot tolerate alkaline conditions. Disappearance of mycorhiza 

 leads to death of the plants, and consequently, the soil must be 

 acid for successful propagation of these species. 



Many orchid seeds germinate normally only in the presence of 

 mycorhizal fungi and were difficult to propagate until it was 

 found that proper nutrient media could compensate for the ab- 

 sence of the fungus. Such evidence indicates that, regardless of 

 what the fungus may take from the root, the vascular plant is 

 benefited by the presence of the mycorhiza or may actually be 

 dependent upon it. Probably the benefit is derived through some 

 nutritional improvement provided by activities of the fungus. 



Nodules— Certain saprophytic bacteria, living free in many soils, 



