18 Professor Macaire on the 



mination, was the action of water on plants. Theodore de 

 Saussure has proved that water is not decomposed by the 

 action of vegetable life, but that its elements are fixed or 

 solidified in the plant, in their state of combination itself. In 

 the process of fermentation this assimilated water may be 

 decomposed, and furnish its oxygen to the carbon, in order to 

 form carbonic acid ; but in no case do plants absorb the 

 hydrogen of water, in order to disengage its oxygen, the de- 

 composition of the carbonic acid being the only source of the 

 oxygen gas exhaled by vegetables. It is, therefore, by the 

 assimilation of the elements of water as a whole, and without 

 the decomposition of this liquid, that plants increase the pro- 

 portions of oxygen and hydrogen they contain ; for, with re- 

 gard to oxygen, the quantity they preserve after the decompo- 

 sition of the carbonic acid, is too small to account for that 

 which enters into their composition. 



"Water and air are not the only elements necessary to the 

 development of vegetables ; and Theodore de Saussure was the 

 first to shew the importance of the solid matters which enter 

 into their composition, and to point out the quarter whence 

 these are derived. He proved that the roots of plants absorb 

 salts and extracts dissolved in water, although in smaller 

 proportion than the water in which they are dissolved. He 

 determined that the roots have the property of making a 

 selection from among the substances contained in these solu- 

 tions, and that they absorb, in preference, substances which 

 render the solutions least viscid. It is to this power of ab- 

 sorption in the roots that we are to ascribe those saline and 

 earthy substances we find in ashes after the combustion of 

 vegetables. In order to obtain a correct idea of the nature 

 and quantity of these ashes, Theodore de Saussure incinerated 

 and analysed the ashes of seventy-nine difi*erent species of 

 plants or parts of vegetables. He found that the proportion 

 of the elements of the ashes almost always bore a relation to 

 that of the elements which constitute the soil. Thus the 

 plants which grew on a siliceous mountain would furnish, all 

 other things being equal, more silica and less lime than the 

 same vegetables from a calcareous mountain. From this it 

 follows that we must seek in the soil for the source of the 



