ABSORPTION OF GASES. 85 



were capable of doing this ; Senebier, Woodhouse and de 

 Saussure on the other hand found that no such absorption 

 took place. Boussingault, and after him Lawes, Gilbert, and 

 Pugh, arrived at the same conclusion as de Saussure but by a 

 different method : instead of determining the composition of 

 the air in a closed receiver, containing the plant, at the begin- 

 ning and at the end of an experiment, they determined the 

 amount of nitrogen in a seed and then allowed a similar seed 

 to germinate under such conditions that no nitrogen except 

 the free nitrogen of the air could have access to it. They 

 ascertained that the amount of the nitrogen in the seedling 

 was not greater than that in the seed, and they therefore con- 

 cluded that the young plant had not taken up this gas from 

 the air. Their investigations will be discussed more fully 

 hereafter in discussing the nature of the food of plants. 



It must, however, be borne in mind that the cell-sap of 

 plants does doubtless hold dissolved in it a certain amount of 

 free nitrogen, since, as we have seen, this gas is soluble to 

 some extent in water. The limit of solubility is soon reached 

 when a plant is growing in the air, and so, if for the purposes 

 of experiment, the plant is then placed in a receiver containing 

 a limited quantity of air, there will be, as de Saussure found, 

 no diminution of the free nitrogen present. There is a very 

 great difference between the relative amounts of oxygen, 

 carbon dioxide, and nitrogen, which are absorbed in the 

 gaseous form by plants. We have seen that under certain 

 circumstances the whole of the oxygen in a limited atmosphere 

 may be absorbed by plants, and this is true also of the carbon 

 dioxide, whereas no nitrogen is absorbed, for the cell-sap of all 

 plants exposed to the air is saturated with this gas. We con- 

 cluded, on account of the large quantities of oxygen and 

 carbon dioxide which are absorbed, that these gases enter 

 into the metabolism of the plant ; we must therefore conclude 

 that, since the amount of free nitrogen absorbed by a plant is 

 so small that it can be accounted for by the mere solubility of 

 the gas, this gas does not enter into the metabolism of the 

 plant. 



