22 



SOIL CONDITIONS AND PLANT GROWTH 



range of temperature nearly up to the death point. At higher tem- 

 peratures it is necessary to work for a short period only, so as to elimi- 

 nate the element of fatigue, but there is no break in the curve. 



For the growth of the plant, however, it is necessary that all the 

 processes should work harmoniously together, and that the protoplasm 

 should remain healthy and vigorous. Now the temperature range 

 over which protoplasm lives and the somewhat delicate adjustment of 

 the processes holds together, is very restricted ; beyond a certain point, 

 further temperature increases do not cause more growth, but throw 

 the adjustment out of gear and act adversely on the protoplasm. 

 Thus we get a bending over of the curve. 



The distinction is well brought out in the work of Brown and 

 Escombe (59) on the influence of varying partial pressures of carbon 

 dioxide on photosynthesis. So long as photosynthesis alone was con- 

 sidered, and other factors eliminated, its amount was proportional to 

 the partial pressure : 



But when plants were grown in atmospheres containing various 

 amounts of carbon dioxide then a wholly different relationship was 

 observed : 1 



Assimilation must have gone on at the accelerated rate in the 

 beginning, but the other processes were unable to keep pace and so 

 they set a limit to the speed of growth. A factor that thus proves 



1 The details of these experiments have been criticised by Demoussy (84), but the 

 general conclusion is probably sound. 



