ABSORPTION OF MINERAL ELEMENTS BY PLANTS 241 



ments of the salts used for these solutions is excluded, be it K, X, 

 or Mg, the plant will not develop; or more exactly it will develop 

 so long as the reserves in the seed allow. Thus it can be said 

 that normally the plant must obtain all these elements from the 

 soil. Carbon alone is not among these elements, in spite of the 

 fact that carbon constitutes almost half of the weight of the dry 

 substance of every plant. Hence it 

 may be concluded that the plant does 

 not obtain its carbon from the soil and 

 does not require for its development 

 carbon-containing constituents of the 

 soil, such as soil carbonates. 



On the contrary, it is not difficult to 

 ascertain that the carbon of the carbon 

 dioxide present in the atmosphere 

 although in quite insignificant amounts, 

 forming only about 0.03 per cent of its 

 volume, still is absolutely indispensable 

 for the nutrition of plants. To show 

 this, place a plant in an atmosphere 

 deprived of carbon dioxide, for instance, 

 under a glass bell jar, into which the air 

 enters through a layer of soda lime, 

 which absorbs carbon dioxide (Fig. 72). 

 A check in the further accumulation of 

 substance will very soon be observed, 

 and the plant will soon show symptoms 

 of starvation. The same effect will be 

 obtained if the roots are immersed in a ^'<>- i'^NaLTokr^':'"''' 

 solution deprived of carbon constitu- 

 ents, or if they remain in a soil rich in humus. In the latter 

 case, special care must be taken to prevent the penetration of 

 carbon dioxide from the soil, where it is continuously being 

 produced by bacterial activity, into the glass bell jar covering 

 the aerial parts of the plant. For this purpose, a container 

 holding a solution of potassium hydroxide may be placed so as 

 to absorb any carbon dioxide that might diffuse out of the soil. 



Both water cultures and experiments with the growth of plants 

 in an atmosphere deprived of carbon dioxide show that the need 

 of the plant for carbon is entirely satisfied by the carbon dioxide 



