Growth Measurements 169 



Average number of branches = 2.3 



No. 136 = 2 



Average length of longest branch = 5 mm. 



No. 136 = 5 mm. 



From these records and from any qualitative notes 

 which may be taken, a picture of the effects of any 

 particular experimental variable can be built up. 

 Presentation of results. It is seldom if ever 

 necessary or desirable to present the numerical 

 data in an experiment in toto. The results can 

 usually be presented better in the form of either 

 histograms where only average results for total 

 passages are desired or curves where temporal 

 trends are to be given. Where a number of simul- 

 taneous experiments are to be compared, the re- 

 sults can be presented in absolute units — mm. or 

 mg. or per cent nitrogen. Where experiments 

 carried out on different tissues or at different 

 times, particularly at different times of year, are 

 to be compared, this is not possible, since the ef- 

 fects of varietal differences or of uncontrolled 

 seasonal variables in the environment may have 

 as great or greater effects on growth rates than 

 do the experimental variables. Since all cultures 

 react on the average in approximately the same 

 way and to the same extent to most variables in 

 the environment, this difficulty can be obviated by 

 setting up a control in each series, treating this 

 control always as the norm, and comparing all 



