32-i University of California Publications in Agricultural Sciences [Vol.3 



that there is considerable variation in this respect, some plants, such 

 as Salix sp., growing and thriving in a soil containing no oxygen. 

 Apparently the limiting concentrations of these gases must be worked 

 out for each plant separately. As to the specific effect of lack of 

 oxygen and excess of carbon dioxide resulting in changes in absorption 

 rate little is definitely known. The first effect seems to be a slowing 

 down of growth, which in turn being ordinarily accompanied by the 

 imbibition (in the case of the embryonic growing regions of the root) 

 of water in considerable amounts, reduces absorption markedly. The 

 exact relation between growth and absorption is not well understood 

 at the present time; but it has been shown by MacDougal 57 and others 

 of the Carnegie Institution that growth of embryonic tissues is mainly 

 accomplished by the imbibition of large quantities of water. It can be 

 readily seen, therefore, that if conditions are unfavorable for growth, 

 imbibition and absorption must necessarily be reduced. 



Another factor which acts in a very similar way to lack of aeration, 

 and one little appreciated up to the present time, is that of soil tem- 

 perature. Every year adds more confirmatory evidence to prove that 

 the temperature relations of physiological processes follow certain 

 typical curves, which seem to be identical or closely related for processes 

 of the same fundamental nature in different organisms. The effects 

 of temperature on physiological processes, both in plants and animals, 

 have been investigated by many workers and in general a modified 

 curve of the Van't Hoff type has been obtained where the most careful 

 work ha.s been done. In such curves several cardinal points can be 

 determined, namely, the minimum temperature at which the process 

 goes on, the maximum temperature beyond which the process no longer 

 continues, and the optimum temperature at which the process is most 

 active. This last term has been superseded by what is known as the 

 maximum rate temperature, representing that temperature above which 

 the rate is ultimately decreased and below which the same occurs. 

 Blackman 58 has shown that the term optimum temperature is in- 

 definite, since at certain temperatures physiological processes are very 

 rapid for a time but then slow down, due to the introduction of a time 

 factor. The maximum rate temperature is that temperature above 

 which a time factor is introduced resulting in an ultimate retardation 

 of the process. 



These cardinal temperatures differ somewhat for different processes 

 but still more markedly do they differ for the same process in different 



57 Ibid., Yearbook 15, 1916. 



ss Optima and Limiting Factors, Ann. Bot., vol. 19 (1905), pp. 281-95. 



