THE ROOT 



95 



details were ever before performed in the same way in 

 the presence of so many thousands of spectators. With 

 great pleasure do I recall one sceptic, a local inhabitant 

 of Nijny, who acknowledged that he used to observe our 

 water cultures day after 

 day with the sinister in- 

 tention of convicting us 

 of quackery ; but in the 

 end he became enthusi- 

 astic and thoroughly con- 

 vinced. 



Our problem is now 

 reduced to this : of all 

 the mass of black mould 

 surrounding the plant, 

 it is only an insignifi- 

 cant pinch of a mixture 

 of certain salts that is 

 at any given moment 

 indispensable for its nu- 

 trition. Now let us find 

 out which of the chemi- 

 cal elements entering 

 into the composition of 

 these salts are essential. 

 It has been necessary for 

 this purpose to make a 

 series of experiments 

 either with white sterile 

 soil into which necessary 

 salts had been introduced 

 described above. 



Here are experiments showing the necessity of 

 nitrogen for plants (fig. 26). We take two flower-pots 

 filled with a soil calcined and washed with an acid, and 

 consequently white and sterile. To one of them the 

 ash of plants has been added, containing all the mineral 



FIG. 26. 



or with culture solutions as 



