ABSORPTION OF CARBON 



9 



as far as weight is concerned, it constitutes almost half of the 

 dry matter of every plant, including those grown in water 

 cultures. It must be concluded, therefore, that plants do not 

 obtain carbon from the soil and that the carbon compounds of the 

 soil are not necessary for its development. On the other hand, it 

 is not difficult to be convinced that carbon dioxide, which is 

 present in the atmosphere and which constitutes on an average 

 but 0.03 per cent of its volume, is indispensable for the nutrition 

 of plants. For this purpose, all that is necessary is to place the 

 plant in an atmosphere which is freed from carbon dioxide, for 

 instance, under a bell jar, into which the air enters through a layer 



oda Lima 



Brick 

 Particles 



Wafer 



Solution 



Fig. 2. — Culture of a plant in an atmosphere free from carbon dioxide (after 



Smith, et al.). 



of soda lime (Fig. 2) which absorbs carbon dioxide. It will soon 

 become apparent that the further accumulation of matter is 

 stopped and that the plant is beginning to show signs of starva- 

 tion. This starvation will be observed no matter whether the 

 roots of the plant are in an artificial nutrient solution devoid of 

 carbon compounds or in a soil rich in humus. In the latter case, 

 however, measures should be taken to prevent the carbon dioxide, 

 which is constantly liberated by the soil as a result of bacterial 

 processes, from penetrating into the bell jar, which covers the 

 aerial parts of the plant. For this purpose, a vessel with potas- 

 sium hydroxide may be placed near the plant. 



Water cultures, as well as experiments of growing plants in 

 air free from carbon dioxide, agree that the carbon require- 



