VIL] 



EARLY THEORIES 



20 1 



carbonic acid supplied to them ; finally, after growth 

 had ceased, the amount of nitrogen in the plant and 

 in the soil was determined. In some cases a known 

 quantity of nitrogenous compounds was supplied as 

 manure ; but all the results went to show that there 

 was no gain of combined nitrogen during growth ; the 

 seed and manure at starting contained as much nitrogen 

 as was found in the plant and soil at the end. 



Similar experiments were carried out with the 

 utmost precautions by the Rothamsted investigators, 

 who likewise found no gain of nitrogen by the plant 

 from the atmosphere. The following results, obtained 

 by Lawes and Gilbert in 1858, will serve to show 

 the agreement between the nitrogen supplied and 

 recovered : 



It has sometimes been objected that the plants in 

 these experiments made such a poor growth as compared 

 with their normal development, in the open air that they 

 never attained their usual power of fixing nitrogen. 

 However, Hellriegel's experiments on plants which 

 were supplied with limited amounts of nitrogen showed 

 that growth is practically proportional to the supply of 

 nitrogen as long as that is below the maximum required 

 by the plant. Field experiments at Rothamsted with 



