THE FOOD OF PLANTS. 1 27 



ing on three small blocks of porcelain , b ; within the bell-jar is a glass 

 dish E, containing water, which stands upon the glass support S\ the 

 water in E serves to moisten the soil in the flower-pot P in which the 

 plant is growing ; by means of the tube z, i more water can be introduced 

 into E, if necessary ; carbon dioxide can be introduced, if necessary, by 

 the tube h. The soil in P is heated to redness before the experiment. 

 The seed is sown in P and is then placed under the bell-jar. A similar 

 seed is taken for analysis, and the proportion of nitrogen which it con- 

 tains is accurately estimated. If now the plant developed from the seed 

 sown in P be found, on analysis, to contain no more nitrogen than the 

 seed analysed, it may be concluded that the plant has not absorbed any 

 nitrogen. Since, under the conditions of the experiment, no supply of 

 nitrogen is offered to the plant except the free nitrogen in the air in the 

 bell-jar, it is evident that none of this can have been taken up by the 

 plant. 



The analysis of one experiment may bs given as an illustration. The 

 seed was that of a Dwarf Runner. 



Nitrogen found in the plant 0*0290 grms. 



soil 0-0033 



flower-pot 0-0017 



0-0340 

 seed 0-0349 



Loss of nitrogen 0-0009 j> 



The nitrogenous compounds absorbed by plants are, in 

 addition to the organic substances mentioned above (p. 124), 

 ammonia and its salts, and nitrates. From the researches of 

 Boussingault it appears that the higher plants flourish best 

 when supplied with nitrogen in the form of nitrates, whereas the 

 lower plants absorb it more readily in the form of ammonia ; 

 in fact Pasteur has shewn that the Yeast-plant cannot assimi- 

 late nitrates. It must be pointed out, however, that Boussin- 

 gault's conclusions are too general. Lehmann has found that 

 many plants flourish better when supplied with ammonia- 

 salts than when supplied with nitrates ; this was well marked 

 in the case of the Tobacco-plant. The Beet is another case 

 in point. Schulze and Urich found a large quantity of 

 nitrates in the roots, shewing that the absorbed nitrates had 

 been altered to only a small extent by the plant, and Coren- 

 winder states that he found the amount of nitrates in the 

 roots to be the same as that which he had supplied to the 



