Growth Measurements 159 



fication of this method, using linear rather than 

 area increments as a criterion of growth, has 

 furnished the most generally satisfactory mea- 

 sure of increase yet devised. When grown in 

 culture, roots do not undergo secondary thicken- 

 ing (White, 1934, 65). As a result, they ordinarily 

 take up a diameter characteristic of the species 

 and of the culture medium within a very few milli- 

 meters of the growing point and retain this diam- 

 eter throughout the length of even very old cul- 

 tures (White, 1932, 63, 1933, 64, 141, 1934, 65). 

 The cross sectional area is thus a constant and 

 can be ignored in comparing measurements, so 

 that length alone becomes an accurate measure of 

 volume and weight. Fiedler (1936, 45) has verfied 

 the fact that the weights and linear increments of 

 root cultures bear a constant relation to one 

 another, by parallel measurements and weighings 

 of a series of cultures of different length. This 

 statement, of course, does not apply to branches 

 at the time of or shortly after their initiation 

 (these always have a smaller diameter than do 

 fully established roots), but it is a well established 

 fact that once a branch has acquired its full indi- 

 viduality or "dominance" (discussed elsewhere) 

 it acquires this characteristic diameter. Measure- 

 ments which record only the linear increment of 

 the central axis will thus give an accurate record 



