IN THE DIVIDING ROOT-TIP CELLS OF THE ONION. 41 



conditions or (6) to the actual destruction of enzymes by these extreme 

 temperatures." 



After executing his experiments in a manner as nearly as possible 

 eliminating these perturbing influences, he finds: 



"Within certain not very wide ranges of temperature the rule of van't Hoff 

 applies to the digestive processes in living cold-blooded vertebrates, the aver- 

 age of eight vahd coefficients being 2.62." 



And, in further interpretation of his results in which the velocity in- 

 crease for a 10° C. temperature-increment varied from 0.93 to 7.81 , he says : 



" Those numbers which are greater than 3.00 indicate that the lower temper- 

 ature of the two temperatures compared exercises a destructive or inhibitive 

 action on the digestive secretions; whereas numbers smaller than 2.00 indicate 

 that the higher temperature of the two temperatures compared likewise 

 inhibits or destroys ferment action." 



It is clear that he regards uncomplicated peptic digestion as a simple 

 and purely chemical process which would, therefore, for moderate tem- 

 peratures, show the characteristic Qio value of from +2.0 to +3.0. 

 For these reasons, of the 13 determinations made 5 were rejected as 

 not valid. His 8 valid coefficients, above mentioned, were determined 

 for temperatures approximating the optimum for peptic digestion in 

 each of the several species experimented with. Thus the cardinal 

 temperature-pomts for the particular activity characteristic of the 

 particular species and individuals used in the experiment and must 

 be taken into account in interpreting temperature-indices based upon 

 physiological systems. 



A single index for two factors. — Livingston^ attacked the problem 

 of physiological constants. As he points out in his investigation, he 

 "takes account of the principle of temperature minima, optima, and 

 maxima." Thus, ''basing the indices upon a physiological rather than 

 an exponential system," he finds ''the van't Hoff-Arrhenius principle, 

 upon which is based the exponential series, appears to hold for the 

 elongation of young maize shoots only for a temperature range from 

 about 20° to about 30° C. (Lehenbauer), and the physiological system 

 is approximately true for all temperatures from 12° to 43°C., at least 

 for the conditions of Lehenbauer's experiments." Subsequently the 

 same author (Livingston) worked out "A single index to represent both 

 moisture and temperature conditions as related to plants."^ 



There is always great difficulty in attributing to an elementary and 

 uncomphcated physiological process the Qio values found in any given 

 measurement, so great in the Uving organism is the interrelation of 

 activities. The analysis must, however, strive to isolate the factors 

 and thus seek data based upon relatively simple processes. Formulas 

 duly weighing each factor can then be synthesized. 



1 Livingston, Burton E. "Physiological temperature-indices for the study of plant growth in 

 relation to climatic conditions." Physiol. Res. 1: No. 8: 399, 1916. 



2 Physiol. Res. 1: No. 9: 421-440, 1916. 



