General Physiology of Plants 185 



in ease of investigation, but by no means third in 

 importance, are its changes in rate of activity as 

 evinced in terms of enzymatic digestion, respira- 

 tion, photosynthesis (if any), etc. 



Mass increase has been used in measuring cul- 

 tures' behavior towards nutrient variables, vita- 

 mins, temperature and light changes, and so on. 

 In this way it has been demonstrated, for example, 

 that, whereas the absolute concentrations of va- 

 rious ions or molecules which give optimal growth 

 rates differ widely for different elements — 0.05 M 

 for sucrose (Bobbins, 1922, 57; Bonner and Addi- 

 cott, 1937, 42; White, 1932, 63), 0.02 M for calcium 

 and magnesium (White, 1932, 141), 0.000,002 M 

 for iron (Figs. 49, 50) (White, 1932, 141 ; Robbins 

 and Schmidt, 1938, 60), and 0.000,000,3 M for thia- 

 min (Robbins and Bartley, 1937, 222 ; Bonner, 1937, 

 198; White, 1937, 231) — the range around these 

 optima which will still permit normal growth is 

 in almost all cases of about the same magnitude, 

 from about 0.5 to 5.0 times the absolute optimal 

 value (White, in press). The capacity of the 

 tissue to adjust its behavior vis a vis these widely 

 different substances appears to be about the same, 

 irrespective of the substance under investigation. 

 These powers of adjustment may be enhanced or 

 reduced according to the existing conditions as 

 regards other variables in the environment with 



