Growth Measurements 157 



Increase in protein nitrogen is the best measure 

 of increase in living material available. Increase 

 in carbohydrate may also give a measure of the 

 activity of the protoplasm. It is, however, a cumu- 

 lative or catalytic process, since any given unit of 

 protoplasm may produce 1 or 10 or 100 units of 

 carbohydrate without itself necessarily either in- 

 creasing or decreasing in amount. Estimation of 

 dry weight has been frequently used as a measure 

 of increase of the total material in a culture. If 

 samples for such determination are taken when 

 the cultures are growing actively, this may give a 

 fairly accurate indication of protoplasmic activ- 

 ity. But, if they are taken in what we term the 

 "differentiating" stage in development, there may 

 easily be a rapid increase in dry weight along 

 with an actual decrease in living material. Dry 

 weight determinations are tedious and time-con- 

 suming, although less so than amino-nitrogen 

 determinations. They can be made with a fair 

 degree of accuracy, the error in good hands prob- 

 ably amounting to about 5 per cent plus or minus 

 the true value. This method of recording increase 

 has been used in plant tissue culture studies to 

 some extent, especially by Robbins and his col- 

 leagues (Robbins, 1922, 57, 177; Robbins and 

 Maneval, 1923, 59, 1924, 258; Robbins and Bartley, 

 1937, 170, 222, 223; Robbins and Schmidt, 1938, 



