32 PHYSIOLOGY OF THE FUNGI 



Graphs. The significance of data is frequently best appreciated when 

 presented in graphical form. A graph reminds one that growth is a con- 

 tinuous function in time, whereas a table may suggest a discontinuous 

 process. Growth curves are especially suited to illustrate the rate and 

 amount of growth as a function of time. In Fig. 2 the growth curves of 

 four fungi illustrate differences among species. Growth curves are 

 equally applicable to the study of a single species under different condi- 

 tions. The points representing the data should be given, so that the 

 reader may see how closely the curve fits the data. 



Three-dimensional graphs may be used to represent the relations among 

 three variables. Three-dimensional graphs take the form of a surface. 

 Rahn (1939) has given concise directions for constructing such graphs and 

 models. Schopfer (1943) has used such graphs to represent the growth of 

 Phycomyces hlakesleeanus with respect to the amount of thiamine and 

 asparagine in the medium as a function of time of incubation (Fig. 33). 



Another way of showing the relations among the variables involves the 

 use of a triangular graph. Such a presentation is effective if one desires, 

 for example, to show the effect of the concentrations of three constituents 

 of a medium upon growth. For examples of the use of triangular graphs 

 see Haenseler (1921) and Pratt and Hok (1946). 



Photographs. The presentation of experimental results is frequently 

 improved by the judicious use of photographs. Photographs are 

 particularly useful in comparing the behavior of fungi under different 

 experimental conditions. The behavior of different species under 

 identical conditions may be effectively compared by the use of photo- 

 graphs. Well-labeled photographs also make excellent permanent 

 records of certain types of experimental results. 



FACTORS AFFECTING GROWTH 



All the separate factors comprising the internal and external environ- 

 ment may affect either the rate or the amount of growth, or both. Among 

 the internal factors are the genetic constitution and the internal modifica- 

 tions due to age and to the previous external environment. While more 

 is known about the external factors which affect growth than about the 

 internal factors, it should always be remembered that the external envi- 

 ronment acts by modifying the internal environment. 



Internal factors. One species differs from another, and even one isolate 

 of a species may differ from another in genetic composition. Many 

 mutations have been produced in the laboratory by the action of X rays, 

 ultraviolet rays, and certain chemicals (see Chaps. 10 and 18). These 

 mutants of a single species produced in the laboratory differ from the 

 parent type in one or more biochemical or morphological characteristics 

 and thus correspond to the different isolates of a species found in nature. 



