Growth Measurements 155 



ideal requirements of scientific presentation. 

 True science rests upon quantitative representa- 

 tions. And quantitative changes can be precisely 

 represented in terms of measurements. It is thus 

 natural that measurement should play a dominant 

 role in tissue culture studies and that the accuracy 

 and reliability of measurement should be a major 

 concern of the student of this subject. 



Living organisms, particularly those which, like 

 most plant materials, are incapable of autonomous 

 movement, give evidence of their reactions to 

 changes in environment chiefly by changes in char- 

 acter or velocity of growth. And growth comes 

 into evidence chiefly through increase either in 

 mass or in complexity of some one or more parts 

 of the living material or its products. Increase 

 can be measured in a variety of ways, depending 

 on the nature of the material to be measured. 



Methods. We are, of course, primarily inter- 

 ested in tissue cultures because they are living 

 materials. Increase of living material, as dis- 

 tinct from non-living secretions, is therefore of 

 the greatest importance. These living materials 

 are chiefly proteins. The carbohydrates and fats 

 and the nitrogenous materials of non-protein 

 character are largely constituents which can be 

 removed without destroying the organism's ca- 

 pacity to live. This is also true of some of the 



