Figure 1-25. An example of commensalism — rootless 

 Spanish moss {Tillandsia usneoides) attached to an old live 

 oak {Quercus virginiana). Brackish marsh is shown in the 

 background. Spanish moss has a wide distribution, from 

 southern Maryland through tropical America to Argentina 

 and Chile. February, Louisiana. (U.S.D.A. Soil Conserva- 

 tion Service.) 



ing, stubble burning, seed inspection, and other agronomic meas- 

 ures. ^°'' Dallisgrass, an important forage plant in Georgia, dies 

 when it is not mowed, evidently because of foliage diseases; but 

 vaseygrass, on the other hand, soon dies when it is mowed or 

 grazed too closely. ^^ 



Several examples of the effects of parasites on the grouping of 

 species will be given. The wood-rotting fungus Fomes ignarius, 

 wood-boring larvae of the beetle Saperda, and the defoliating 

 beetle Phytodecta americana place the aspen at a disadvantage in 



52 • Species aimd Popvalatioxms 



