Growth Measurements 161 



duced into dry weight measurements by hygro- 

 scopic absorption of moisture and is less than 

 errors likely to arise from inaccuracies of mea- 

 surements (see below), so that it is not important. 

 Moreover, this error is always a negative one, the 

 measurements will (aside from errors of manipu- 

 lation) always be minimal ones, with the result 

 that every experiment recorded in this way is 

 slightly weighted against the experimenter. This 

 has thus furnished a very satisfactory criterion 

 of growth. 



The usefulness of any method depends, of 

 course, largely on its simplicity, accuracy, and 

 degree of reliability. The method used in making 

 measurements is extremely simple (Fig. 40). The 

 flask containing the culture to be measured is held 

 in the left hand. A flexible celluloid rule is used 

 to make the measurement. If the culture is very 

 short (under 30 mm.), it can be caused to float 

 against the glass so that both ends touch and the 

 ruler is laid alongside. For cultures more than 

 30 mm. long, particularly if they have curved 

 greatly, it is best to place the rule below the flask 

 and bend it to approximate the curve of the root. 

 One must be careful to look through the glass 

 downward (parallel to the straight component of 

 the sides) rather than sidewise (radially to the 

 curved component) so that the curvatures will not 



