200 



SCIENCE 



[N. S. Vol. XL VI. No. 1183 



application of the Law of the Minimum 

 has been worked out in many cases and has 

 been of great use in the interpretation of 

 complicated relations ; but it has been rec- 

 ognized as a law and has been consciously 

 applied by plant physiologists and physio- 

 logical chemists only.^ Without doubt it 

 can be used to advantage in many problems 

 of the physiology, morphology and ecology 

 of both plants and animals. 



The Law of the Minimum must be taken 

 into account in all experimental work, for 

 which it serves both as a precaution and a 

 guide.^ "When investigating the effect of 

 an external factor such as temperature, 

 light, etc., on any given process, it is neces- 

 sary to keep all other variable factors con- 

 stant, and then to determine the eifect of 

 changes in the factor under consideration. 

 What results might be obtained when this 

 method is used in studying carbon assimila- 

 tion? Suppose the CO, supply and the 

 light are kept constant, while the tempera- 

 ture is varied. If the CO, supply is such 

 that it becomes a limiting factor when the 

 temperature rises above 10° C. then the 

 rate of assimilation will rise with the tem- 

 perature up to this point, but will remain 

 constant at all higher temperatures, until 

 the destructive effect of the high tempera- 

 ture is manifested and the curve again falls 

 off. Above 10° C. variations in the tem- 

 perature have no apparent effect under 

 these experimental conditions. But if the 

 CO2 supply is increased so as to permit 

 more rapid assimilation, then the tempera- 

 ture curve can be extended. Negative re- 

 sults from such an experimental method 

 are therefore without significance. It is not 

 enough that the experiment be conducted 

 under constant conditions; the constant 

 factors must not interfere in any way with 

 the carrying out of the process ; that is, they 



8C/. tte work of L. B. Mendel, T. B. Osborne 

 and their pupils. 



9 Cf. B. E. Livingston, loc. cit. 



must not be limiting factors. On the other 

 hand, it is a simple matter to determine by 

 the shape of the curve whether any other 

 factor than the one under investigation is 

 a limiting factor. Such is always the case 

 when a break occurs in the curve; usually 

 the curve rises at first and later runs paral- 

 lel with X-axis. Such curves were obtained 

 by Miss Matthaei^" in studying the 

 carbon assimilation of cherry laurel at 

 varying temperatures with unit light inten- 

 sity. The problem is much more compli- 

 cated, however, when variation of one factor 

 is accompanied by changes in one or more 

 other factors. This complication arises in 

 plotting the temperature curve for enzyme 

 activity. The curve rises at first according 

 to van't Hoff's law of reactions, but even- 

 tually a maximum value is reached and the 

 curve falls off. At some point near the end 

 of the ascending portion of the curve a 



Fig. 2. Effect of temperature on the activity of 

 malt diastase. (After Kjeldahl.) 



break occurs: for all temperatures below 

 this point, temperature is the limiting fac- 

 tor and determines the activity of the 

 enzyme; for all temperatures above this 

 point, not temperature, but the amount of 

 enzyme is the limiting factor. The higher 

 temperatures cause a permanent inactiva- 

 tion or decomposition of the enzyme so that 

 its activity is conditioned only secondarily 

 by the temperature. There is also a time 

 factor involved here ; the longer the tem- 

 perature acts, the more the enzyme is de- 

 composed, within certain limits. The study 



loPML Trans., B, 196: 47-105, 1904. 



