ASSIMILATION OF CARBON BY PLANTS 201 



why this method is appHcable on a wide scale only in places 

 where electric energy is very cheap, for instance, close to large 

 hydroelectric stations, and only to the more expensive crops, 

 e.g., in the growing of vegetables rich in vitamins in polar regions 

 or for scientific purposes. 



Considerably less expensive and more profitable is the addition 

 of artificial light to normal daylight during the winter months, 

 when the day is too short and the light intensity too weak. In 

 this case, smaller intensities of electric light applied during shorter 

 periods suffice. Besides, several hours of natural daylight con- 

 tribute to a more adequate formation of the plants. It is in the 

 form of additional illumination that electric light is being widely 

 applied in greenhouse culture and for scientific purposes. With 

 such an addition, flow^ers and vegetables may be cultivated during 

 the darkest winter months, and two to three and even four 

 generations of crop plants instead of one may be obtained in the 

 course of one year. This is of major importance for accelerating 

 the production and selection of valuable hybrids. 



46. Dependence of Photosynthesis on the Amount of Carbon 

 Dioxide. The Carbon Dioxide Cycle in Nature. — The gradual 

 lag in increase of photosynthesis in proportion to increased 

 intensity of light depends not only on the fact that the chloro- 

 plasts are not able to use the excess of light energy, but also on 

 the fact that the carbon dioxide absorbed by them per unit time 

 is not sufficient for the full utilization of this energy. An 

 increase of the carbon dioxide content in the atmosphere, there- 

 fore, always leads to greater photosynthesis. This may be seen 

 from Fig. 61, which shows the increase in assimilation of pine 

 needles, with the increase of the carbon dioxide content from 0.03 

 per cent, the normal amount in the air, to 0.28 per cent. It must be 

 noted that the curve expressing the dependence of photosynthesis 

 on the carbon dioxide content is of a logarithmic t3T)e, as was 

 also the curve showing the dependence on light, and that this 

 is the usual form of curves expressing the relationship of different 

 physiological processes to the intensity of external factors. 



Numerous investigations that have been conducted by different 

 authorities seem to agree that the ''normal" or ''natural" content 

 of the carbon dioxide of the air, viz., about 0.03 per cent, is by 

 no means the optimum but rather the minimum amount for the 

 plant and that an increase of carbon dioxide often appears favor- 



