THE METABOLISM OF PLANTS. 2Q5 



not quite identical. In the first place, the minimum tempera- 

 ture for respiration (including both the absorption of oxygen 

 and the exhalation of carbon dioxide) is lower than that for 

 growth. It is true that growth may take place at very low 

 temperatures, but this does not affect the truth of the general 

 statement just made. In the second place the optimum tem- 

 perature for growth is reached much earlier than that for 

 respiration (Rischawi, Mayer); that is, that in the case of any 

 given plant, the optimum temperature for respiration lies very 

 near the maximum temperature which is compatible with the 

 maintenance of life, whereas the optimum temperature for 

 growth lies considerably below this limit. If these facts were 

 expressed graphically, we should find that the curve of growth 

 begins to rise rather later than the curve of respiration, and 

 that the former begins to fall whilst the latter still continues 

 to rise. 



In endeavouring to account for the difference in the curves 

 of growth and of respiration, we can readily explain why 

 they do not begin to rise at the same point, in other words, 

 why the minima of temperature are not the same in the two 

 cases. Growth, we have already seen, can only take place 

 when the evolution of energy in the plant is considerable ; 

 at the minimum temperature for respiration the destructive 

 metabolism is feeble, too feeble to admit of growth ; it is there- 

 fore only at a higher temperature, a temperature at which 

 destructive metabolism is more active, that growth begins. 

 The only explanation which we can offer of the fact that the 

 curve of growth falls at temperatures at which the curve of 

 respiration continues to rise, is this, that the process of growth 

 itself is affected prejudicially by high temperatures. At these 

 temperatures the evolution of energy is adequate, but the 

 processes upon which increase in bulk directly depends are in 

 some way interfered with. 



We see, then, that though growth is dependent upon an 

 evolution of energy, that is, upon destructive metabolism, it is 

 nevertheless a distinct process which is not always affected by 

 variations in external conditions in the same way as destruc- 

 tive metabolism ; in fact, it may be affected by conditions 



