156 * BIOLOGY OF PLANTS. 



3. Carbonic acid contains sixteen parts in twenty-two of 

 oxygen, and is a further source of this substance. 



4. Gcinc or humus contains oxygen, which in the processes 

 of vegetation, is brought into contact with the vegetable or- 

 gans, and may thus be the source of a portion of the oxygen 

 which plants contain. 



5. Nitric acid may be still another source, and perhaps 

 several other acids, as the carbonic, phosphoric and sulphu- 

 ric, which are known to exist in all soils. 



IV. Theory of the A ssimilation of Oxygen and of Hydro- 

 gen. Woody fibre, which is the solid part of plants, and is 

 the most abundant of the products of vegetables, is composed 

 of carbon, with oxygen and hydrogen in the proportions to 

 form water ; that is, if the hydrogen and oxygen were to com- 

 bine, water would be formed and carbon left in a free state ; 

 or the composition may be represented by the elements of 

 carbonic acid, with a certain quantity of hydrogen. 



Now the wood may be formed by the decomposition of 

 carbonic acid ; the carbon uniting with the elements of water, 

 and the oxygen escaping as effete matter ; or, what is more 

 probable, the carbonic acid may combine with the hydrogen 

 of the decomposed water, while the oxygen of the water 

 escapes. In either case, the quantity of oxygen separated 

 would be exactly the same. 



But there are other compounds (as the acids), in which oxy- 

 gen is in excess ; in the process of their formation, therefore, 

 less oxygen would be separated. In case oxygen is in less 

 quantity than in the relative proportion to form water, as it 

 is in alkalies and neutral substances, such as starch, sugar, 

 wax and all resinous bodies, then, in the processes of assimi- 

 lation, much more oxygen would be separated. Such sub- 

 stances yield a larger quantity, or all of the oxygen, both of 

 the carbonic acid and of the water.* If this is a true rep- 



* The following table of Liebig, illustrates still further the changes 



