158 Plant Tissue Culture 



60, 1939, 61, 224), by Gautheret (1939, 285) and 

 by Malyschev (1932, 53, 54). Like ammo-nitro- 

 gen determinations, however, it involves destruc- 

 tion of the specimens studied, discontinuity of 

 observations, and the use of aliquots. 



There is, however, a third method which does 

 not involve destruction of the tissue, permits con- 

 secutive measurements, and in practiced hands is 

 at least as accurate as is the measurement of dry 

 weights. That is the measurement of area in- 

 creases. In studying animal tissue cultures, it has 

 long been the practice to project the image of a 

 culture on paper at hourly, daily, or weekly inter- 

 vals, making tracings of the outline, to measure 

 the areas of these tracings with a planimeter, and 

 from the results to construct curves represent- 

 ing the growth increments (Ebeling, 1921, 456; 

 Parker, 1938, 26). It is recognized that this 

 method may err in making no distinction between 

 cell migration and cell increase and in taking no 

 account of changes in thickness of the cultures. 

 The first objection does not apply to the use of this 

 method for plant cultures, since no migration 

 takes place from plant tissues. The second objec- 

 tion is valid when the areas of callus cultures are 

 under investigation, since these ordinarily develop 

 as somewhat flattened hemispheres of tissue. 



When applied to root cultures, however, a modi- 



