SAPROPHYTISM AND SYMBIOSIS 759 



are unicellular, filamentous, and colony-forming species. The bacterial 

 form, Leuconostoc, resembles the algal form, Nostoc (fig. 8), both in gen- 

 eral appearance and in behavior. Similarly, the alga groups, Siphonales 

 and Conjugales, show many resemblances to the fungus groups, Oomy- 

 cetes and Zygomycetes, respectively (see Part I). 



It is not difficult to picture the probable stages in the development of 

 holosaprophytism and to imagine some of the possible underlying causes. 

 Since the lower algae are mixophytes, saprophytism does not seem to in- 

 volve the introduction of a new character, but chiefly the elimination of 

 autophytism; probably there develops also an increased capacity for the 

 utilization of organic food. The experiments previously cited appear to 

 suggest that food-making by chlorophyll may be diminished or even 

 checked in the presence of an excess of soluble carbohydrates. A sec- 

 ond stage appears to be the gradual disappearance of chlorophyll, which 

 also is associated with an abundant external food supply; this stage 

 indicates the passage from partial saprophytism to temporary holo- 

 saprophytism. The final stage, the elimination of the possibility of 

 chlorophyll formation and hence of autophytism, may be occasioned by 

 the disappearance of plastids, but the factors involved in this are at 

 present quite unknown. 



Progressive variability in saprophytes. While most saprophytic 

 fungi and bacteria can be grown in various culture media, a sudden 

 transfer to an unaccustomed medium results frequently in impaired 

 activity or in death. However, if the change is made gradually through 

 a series of intermediate solutions, successful cultures may result. For 

 example, Bacillus fluorescens pulridus ordinarily has its optimum growth 

 conditions at 22 C, and can scarcely grow above 35 C; however, if 

 media of gradually increasing temperature are used, a race is developed 

 that grows vigorously at 41 C. Thermal bacteria, with an optimum of 

 37 C, can be treated similarly until they thrive at 66 C. Likewise, many 

 saprophytes can be grown in toxic media if the poisons are increased grad- 

 ually in successive cultures; in this way Penicillium can be grown ul- 

 timately in concentrated solutions of copper sulfate. Many forms, as 

 Aspergillus and Penicillium, can be cultivated utlimately in salt solu- 

 tions that are so highly concentrated as to produce plasmolysis or death 

 if the transfer from ordinary media is made suddenly. Some forms 

 secrete enzyms whose character differs with the nature of the substratum, 

 Penicillium even secreting a wood-destroying enzym, hadromase, when 

 grown on wood. Anaerobic bacteria may be accustomed "gradually to 



