42 



DURATION OF THj: SEVERAL MITOTIC STAGES 



(d) DIFFERENCE BETWEEN PHYSIOLOGICAL AND PURELY CHEMICAL 

 TEMPERATURE-VELOCITY REACTIONS. 



Physiological processes. — Harvey/ in his inves- 

 tigations of the rate of conduction of the nerve 

 impulse in the medusa Cassiopea, calls attention 

 to the fact that within medium temperatures — 

 that is, from 18° to 38° C. — the velocity-increment 

 per definite temperature-rise for physiological 

 processes declines as the temperature increases, 

 whereas in purely chemical reactions the velocity- 

 increment increases as the temperature rises. He 

 gives the accompanying table showing the former 

 principle for the experiment above named. 



In interpreting this behavior Harvey says: 



"If the rate of nerve conduction depends on the velocity of some chemical 

 reaction in the nerve, the above-mentioned difference in its temperature curve 

 remains to be explained. It is possible, indeed probable, that yet another 

 factor than reaction velocity determines conduction rate, and the resultant 



curve of the two factors is the one actually observed Different enzymes 



exhibit maxima at different temperatures. Most of these are rather high, 

 much higher than the maximum for nerve-conduction, which lies at about 

 33° C. The same ferment obtained from different sources may exhibit dif- 

 ferent maxima .... we may say that the propagation of the nerve impulse 

 is not only dependent on the velocity of a chemical reaction, hut that the reac- 

 tion is further accelerated hy the presence of an enzijme. Thus the characteristic 

 difference in the form of curve from that of a simple reaction." 



Growth or permanent hulk increase. — Lehenbaur,^ presents the table 

 shown herewith. The purpose of his experiments was to test the appli- 

 cabihty of van't Hoff's principle to the rate of growth in the stem- 

 shoots of maize seedlings. He points 

 out that his results approximate van't 

 Hoff's law in the medium temperatures 

 only, that is, from 20° to 30° C, where 

 the concomitant temperature-coefficients 

 range from H-1.88 to +2.40. The table 

 is indeed a most interesting one, for 

 growth alone is considered, and this he 

 studied in its more restricted sense, 

 namely, permanent increase in bulk 

 disregarding mitotic activity. There is 

 no constant velocity-increment with each 

 temperature-rise of 10° C, but it will be 



1 Harvey, E. Newton. "Effects of different temperatures on the medusa Cassiopea, with 

 special reference to the rate of conduction of the nerve inpulse," Carnegie Inst. Wash. Pub. No. 

 132, pp. 27-39, 1910. 



2 Lehenbauer, Philip A. "Growth of maize seedlings in relation to temperature." Physiol. 

 Res. i:No. 5:281, 1914. 



