172 



INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



after 72 hours, whereas in a 24-hour cycle they presumably would all 

 divide after twice 24 hours. Therefore any departure from the normal 24- 

 hour cycle, or from the length of cycle induced by abnormal temperatures, 

 will disrupt this synchronization of cell divisions in the growing point and 

 cause irregularities in growth. In this way thermoperiodicity has a func- 

 tion like hormones in the coordination of growth. These considerations 

 approach rather closely the work reported recently on synchronization of 

 cell divisions in microorganisms, wdiere discreet changes in conditions may 

 cause all cells to divide simultaneously. This induced rhythm will gradu- 

 ally be lost, unless new inducing conditions are applied. 



1.0- 



OB- 



0.6 



0.4 



0.2 



O- 



-0.2 



\ ^ ^Maximal growth rat« 



\ \^ 1 p«» stems 



\ \ ^ 



\ \ V*—* streaminq 



\ \ \ 



Node development 

 pea stems 



^J-Sugar \ \ \ 



/ tr«nsloc»tion \ '^ 



£--^ \S— Growth r«t« 



rmtf^ \ \ intact tomatoes 



Fig. 9. Arrhenius plots of 

 temperature dependence of var- 

 ious processes, presented in figs 

 1, 2, 3 and 4. Abscissa: inverse 

 of absolute temperature; ordi- 

 nate: logarithm of the rate of 

 the process. 



Growth 

 excised tomato 

 roots 



\\ 



000330 0.00340 000350 0.00360 



As a historical note it can be mentioned that in the time of de Candolle 

 100 years ago, temperature was considered a major factor in the effect of 

 climate on vegetation. It is rather remarkable that the primitive concept of 

 de Candolle (4) has not undergone any development in connection with 

 newer knowledge, but that among agroclimatologists the 'heat sum' is 

 still connnonly used. This concept presupposes a direct proportionality 

 between temperature and development of a crop plant, more or less like 

 the effect of temperature on protoplasmic streaming. From figures 1 and 2 

 it can be seen that only over a very short range, growth of tomatoes (and 

 other plants) is more or less proportional with temperature. Therefore a 

 'heat sum' is impermissible since it adds all effects of temperature, whereas 

 above the optimal temperature these effects rather should be subtracted. 

 Or, more accurately, the temperature curve of most plants can be repre- 

 sented by a cubic equation. 



