ECOLOGY 



93 



phyll, have no power of photosynthesis and consequently bear 

 no necessary light relation. The group includes chiefly fungi, 

 but there are not a few flowering plants which have degenerated 

 to the condition of saprophytes. 



215. What has been said of saprophytes might be repeated 

 for the group called parasites; excepting this, that they live in 



FIG. 48. Section ol a lichen. Near the upper surface are groups of rounded 

 cells (shaded). These are algal cells, arranged in groups by fission. The re- 

 maining parts are formed by the filaments of the fungus. (From Sayre after 

 Sachs.) 



or upon the tissues of living organisms and the result of their 

 activity is called disease. Most infectious diseases of both 

 animals and plants are to be ascribed to this class of organisms. 

 A few chlorophyll-bearing plants have a parasitic habit and 

 hence are called partial parasites. Facultative parasites may 

 live either as true parasites or as saprophytes, while obligate 

 parasites can exist only as parasites. 



